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<title>The Boost Parameter Library</title>
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<body>
<div class="document" id="the-boost-parameter-library">
<h1 class="title">The Boost Parameter Library</h1>

<p><a class="reference external" href="../../../../index.htm"><img alt="Boost" src="../../../../boost.png" /></a></p>
<hr class="docutils" />
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Abstract:</th><td class="field-body">Use this library to write functions and class templates that can
accept arguments by name:</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
new_window(
    &quot;alert&quot;
  , <strong>_width=10</strong>
  , <strong>_titlebar=false</strong>
);

smart_ptr&lt;
    Foo
  , <strong>deleter&lt;Deallocate&lt;Foo&gt; &gt;</strong>
  , <strong>copy_policy&lt;DeepCopy&gt;</strong>
&gt; p(new Foo);
</pre>
<p>Since named arguments can be passed in any order, they are especially useful
when a function or template has more than one parameter with a useful default
value.  The library also supports <em>deduced</em> parameters: that is to say,
parameters whose identity can be deduced from their types.</p>
<!-- @jam_prefix.append('''
project test
    : requirements <include>. <implicit-dependency>/boost//headers ;
''') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace test {

    BOOST_PARAMETER_NAME(title)
    BOOST_PARAMETER_NAME(width)
    BOOST_PARAMETER_NAME(titlebar)

    BOOST_PARAMETER_FUNCTION(
        (int), new_window, tag, (required (title,*)(width,*)(titlebar,*))
    )
    {
        return 0;
    }

    BOOST_PARAMETER_TEMPLATE_KEYWORD(deleter)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(copy_policy)

    template <typename T>
    struct Deallocate
    {
    };

    struct DeepCopy
    {
    };

    namespace parameter = boost::parameter;

    struct Foo
    {
    };

    template <typename T, typename A0, typename A1>
    struct smart_ptr
    {
        smart_ptr(Foo*);
    };
}
using namespace test;
int x =
'''); -->
<!-- @test('compile') -->
<hr class="docutils" />
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Authors:</th><td class="field-body">David Abrahams, Daniel Wallin</td>
</tr>
<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:daniel&#64;boostpro.com">daniel&#64;boostpro.com</a></td>
</tr>
<tr class="field"><th class="field-name">organization:</th><td class="field-body"><a class="reference external" href="http://www.boostpro.com">BoostPro Computing</a></td>
</tr>
<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date: 2005/07/17 19:53:01 $</td>
</tr>
<tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Daniel Wallin
2005-2009. Distributed under the Boost Software License,
Version 1.0. (See accompanying file LICENSE_1_0.txt
or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td>
</tr>
</tbody>
</table>
<hr class="docutils" />
<p>[Note: this tutorial does not cover all details of the library.  Please see
also the <a class="reference external" href="reference.html">reference documentation</a>]</p>
<div class="contents topic" id="table-of-contents">
<p class="topic-title first"><strong>Table of Contents</strong></p>
<ul class="auto-toc simple">
<li><a class="reference internal" href="#motivation" id="id24">1&nbsp;&nbsp;&nbsp;Motivation</a><ul class="auto-toc">
<li><a class="reference internal" href="#named-function-parameters" id="id25">1.1&nbsp;&nbsp;&nbsp;Named Function Parameters</a></li>
<li><a class="reference internal" href="#deduced-function-parameters" id="id26">1.2&nbsp;&nbsp;&nbsp;Deduced Function Parameters</a></li>
<li><a class="reference internal" href="#class-template-parameter-support" id="id27">1.3&nbsp;&nbsp;&nbsp;Class Template Parameter Support</a></li>
</ul>
</li>
<li><a class="reference internal" href="#tutorial" id="id28">2&nbsp;&nbsp;&nbsp;Tutorial</a><ul class="auto-toc">
<li><a class="reference internal" href="#parameter-enabled-functions" id="id29">2.1&nbsp;&nbsp;&nbsp;Parameter-Enabled Functions</a></li>
<li><a class="reference internal" href="#parameter-enabled-member-functions" id="id30">2.2&nbsp;&nbsp;&nbsp;Parameter-Enabled Member Functions</a></li>
<li><a class="reference internal" href="#parameter-enabled-function-call-operators" id="id31">2.3&nbsp;&nbsp;&nbsp;Parameter-Enabled Function Call Operators</a></li>
<li><a class="reference internal" href="#parameter-enabled-constructors" id="id32">2.4&nbsp;&nbsp;&nbsp;Parameter-Enabled Constructors</a></li>
<li><a class="reference internal" href="#parameter-enabled-class-templates" id="id33">2.5&nbsp;&nbsp;&nbsp;Parameter-Enabled Class Templates</a></li>
</ul>
</li>
<li><a class="reference internal" href="#advanced-topics" id="id34">3&nbsp;&nbsp;&nbsp;Advanced Topics</a><ul class="auto-toc">
<li><a class="reference internal" href="#fine-grained-name-control" id="id35">3.1&nbsp;&nbsp;&nbsp;Fine-Grained Name Control</a></li>
<li><a class="reference internal" href="#more-argumentpacks" id="id36">3.2&nbsp;&nbsp;&nbsp;More <span class="concept">ArgumentPack</span>s</a></li>
</ul>
</li>
<li><a class="reference internal" href="#best-practices" id="id37">4&nbsp;&nbsp;&nbsp;Best Practices</a><ul class="auto-toc">
<li><a class="reference internal" href="#keyword-naming" id="id38">4.1&nbsp;&nbsp;&nbsp;Keyword Naming</a></li>
<li><a class="reference internal" href="#namespaces" id="id39">4.2&nbsp;&nbsp;&nbsp;Namespaces</a></li>
<li><a class="reference internal" href="#documentation" id="id40">4.3&nbsp;&nbsp;&nbsp;Documentation</a></li>
</ul>
</li>
<li><a class="reference internal" href="#portability-considerations" id="id41">5&nbsp;&nbsp;&nbsp;Portability Considerations</a><ul class="auto-toc">
<li><a class="reference internal" href="#perfect-forwarding-support" id="id42">5.1&nbsp;&nbsp;&nbsp;Perfect Forwarding Support</a></li>
<li><a class="reference internal" href="#boost-mp11-support" id="id43">5.2&nbsp;&nbsp;&nbsp;Boost.MP11 Support</a></li>
<li><a class="reference internal" href="#no-sfinae-support" id="id44">5.3&nbsp;&nbsp;&nbsp;No SFINAE Support</a></li>
<li><a class="reference internal" href="#no-support-for-result-of" id="id45">5.4&nbsp;&nbsp;&nbsp;No Support for <tt class="docutils literal">result_of</tt></a></li>
<li><a class="reference internal" href="#compiler-can-t-see-references-in-unnamed-namespace" id="id46">5.5&nbsp;&nbsp;&nbsp;Compiler Can't See References In Unnamed Namespace</a></li>
</ul>
</li>
<li><a class="reference internal" href="#python-binding" id="id47">6&nbsp;&nbsp;&nbsp;Python Binding</a></li>
<li><a class="reference internal" href="#reference" id="id48">7&nbsp;&nbsp;&nbsp;Reference</a></li>
<li><a class="reference internal" href="#glossary" id="id49">8&nbsp;&nbsp;&nbsp;Glossary</a><ul class="auto-toc">
<li><a class="reference internal" href="#argument-or-actual-argument" id="id50">8.1&nbsp;&nbsp;&nbsp;Argument (or “actual argument”)</a></li>
<li><a class="reference internal" href="#parameter-or-formal-parameter" id="id51">8.2&nbsp;&nbsp;&nbsp;Parameter (or “formal parameter”)</a></li>
</ul>
</li>
<li><a class="reference internal" href="#acknowledgements" id="id52">9&nbsp;&nbsp;&nbsp;Acknowledgements</a></li>
</ul>
</div>
<hr class="docutils" />
<div class="section" id="motivation">
<h1><a class="toc-backref" href="#id24">1&nbsp;&nbsp;&nbsp;Motivation</a></h1>
<p>In C++, <a class="reference internal" href="#arguments">arguments</a> are normally given meaning by their positions with respect
to a <a class="reference internal" href="#parameter">parameter</a> list: the first argument passed maps onto the first parameter
in a function's definition, and so on.  That protocol is fine when there is at
most one parameter with a default value, but when there are even a few useful
defaults, the positional interface becomes burdensome:</p>
<ul>
<li><div class="first compound">
<p class="compound-first">Since an argument's meaning is given by its position, we have to choose an
(often arbitrary) order for parameters with default values, making some
combinations of defaults unusable:</p>
<pre class="compound-middle literal-block">
window* new_window(
    char const* name
  , <strong>int border_width = default_border_width</strong>
  , bool movable = true
  , bool initially_visible = true
);

bool const movability = false;
window* w = new_window(&quot;alert box&quot;, movability);
</pre>
<p class="compound-middle">In the example above we wanted to make an unmoveable window with a default
<tt class="docutils literal">border_width</tt>, but instead we got a moveable window with a
<tt class="docutils literal">border_width</tt> of zero.  To get the desired effect, we'd need to write:</p>
<pre class="compound-last literal-block">
window* w = new_window(
    &quot;alert box&quot;, <strong>default_border_width</strong>, movability
);
</pre>
</div>
</li>
<li><div class="first compound">
<p class="compound-first">It can become difficult for readers to understand the meaning of arguments
at the call site:</p>
<pre class="compound-middle literal-block">
window* w = new_window(&quot;alert&quot;, 1, true, false);
</pre>
<p class="compound-last">Is this window moveable and initially invisible, or unmoveable and
initially visible?  The reader needs to remember the order of arguments to
be sure.</p>
</div>
</li>
<li><p class="first">The author of the call may not remember the order of the arguments either,
leading to hard-to-find bugs.</p>
</li>
</ul>
<!-- @ignore(3) -->
<div class="section" id="named-function-parameters">
<h2><a class="toc-backref" href="#id25">1.1&nbsp;&nbsp;&nbsp;Named Function Parameters</a></h2>
<div class="compound">
<p class="compound-first">This library addresses the problems outlined above by associating each
parameter name with a keyword object.  Now users can identify arguments by
name, rather than by position:</p>
<pre class="compound-last literal-block">
window* w = new_window(
    &quot;alert box&quot;
  , <strong>movable_=</strong>false
); // OK!
</pre>
</div>
<!-- @ignore() -->
</div>
<div class="section" id="deduced-function-parameters">
<h2><a class="toc-backref" href="#id26">1.2&nbsp;&nbsp;&nbsp;Deduced Function Parameters</a></h2>
<div class="compound">
<p class="compound-first">A <strong>deduced parameter</strong> can be passed in any position <em>without</em> supplying
an explicit parameter name.  It's not uncommon for a function to have
parameters that can be uniquely identified based on the types of arguments
passed.  The <tt class="docutils literal">name</tt> parameter to <tt class="docutils literal">new_window</tt> is one such
example.  None of the other arguments, if valid, can reasonably be
converted to a <tt class="docutils literal">char const*</tt>.  With a deduced parameter interface, we
could pass the window name in <em>any</em> argument position without causing
ambiguity:</p>
<pre class="compound-middle literal-block">
window* w = new_window(
    movable_=false
  , <strong>&quot;alert box&quot;</strong>
); // OK!
window* w = new_window(
    <strong>&quot;alert box&quot;</strong>
  , movable_=false
); // OK!
</pre>
<p class="compound-last">Appropriately used, a deduced parameter interface can free the user of the
burden of even remembering the formal parameter names.</p>
</div>
<!-- @ignore() -->
</div>
<div class="section" id="class-template-parameter-support">
<h2><a class="toc-backref" href="#id27">1.3&nbsp;&nbsp;&nbsp;Class Template Parameter Support</a></h2>
<div class="compound">
<p class="compound-first">The reasoning we've given for named and deduced parameter interfaces
applies equally well to class templates as it does to functions.  Using
the Parameter library, we can create interfaces that allow template
arguments (in this case <tt class="docutils literal">shared</tt> and <tt class="docutils literal">Client</tt>) to be explicitly named,
like this:</p>
<pre class="compound-middle literal-block">
smart_ptr&lt;
    <strong>ownership&lt;shared&gt;</strong>
  , <strong>value_type&lt;Client&gt;</strong>
&gt; p;
</pre>
<p class="compound-middle">The syntax for passing named template arguments is not quite as natural as
it is for function arguments (ideally, we'd be able to write
<tt class="docutils literal">smart_ptr&lt;ownership = shared, …&gt;</tt>).  This small syntactic deficiency
makes deduced parameters an especially big win when used with class
templates:</p>
<pre class="compound-last literal-block">
// <em>p and q could be equivalent, given a deduced</em>
// <em>parameter interface.</em>
smart_ptr&lt;<strong>shared</strong>, <strong>Client</strong>&gt; p;
smart_ptr&lt;<strong>Client</strong>, <strong>shared</strong>&gt; q;
</pre>
</div>
<!-- @ignore(2) -->
</div>
</div>
<div class="section" id="tutorial">
<h1><a class="toc-backref" href="#id28">2&nbsp;&nbsp;&nbsp;Tutorial</a></h1>
<p>This tutorial shows all the basics—how to build both named- and
deduced-parameter interfaces to function templates and class
templates—and several more advanced idioms as well.</p>
<div class="section" id="parameter-enabled-functions">
<h2><a class="toc-backref" href="#id29">2.1&nbsp;&nbsp;&nbsp;Parameter-Enabled Functions</a></h2>
<p>In this section we'll show how the Parameter library can be used to
build an expressive interface to the <a class="reference external" href="../../../graph/doc/index.html">Boost Graph library</a>'s
<a class="reference external" href="../../../graph/doc/depth_first_search.html"><tt class="docutils literal">depth_first_search</tt></a> algorithm.<a class="footnote-reference" href="#old-interface" id="id3"><sup>1</sup></a></p>
<!-- Revisit this

After laying some groundwork and describing the algorithm's abstract
interface, we'll show you how to build a basic implementation with keyword
support.  Then we'll add support for default arguments and we'll gradually
refine the implementation with syntax improvements.  Finally we'll show
how to streamline the implementation of named parameter interfaces,
improve their participation in overload resolution, and optimize their
runtime efficiency. -->
<div class="section" id="headers-and-namespaces">
<h3>2.1.1&nbsp;&nbsp;&nbsp;Headers And Namespaces</h3>
<p>Most components of the Parameter library are declared in a header named for
the component.  For example,</p>
<pre class="literal-block">
#include &lt;boost/parameter/keyword.hpp&gt;
</pre>
<p>will ensure <tt class="docutils literal"><span class="pre">boost::parameter::keyword</span></tt> is known to the compiler.  There
is also a combined header, <tt class="docutils literal">boost/parameter.hpp</tt>, that includes most of
the library's components.  For the the rest of this tutorial, unless we
say otherwise, you can use the rule above to figure out which header to
<tt class="docutils literal">#include</tt> to access any given component of the library.</p>
<!-- @example.append('''
using boost::parameter::keyword;
''') -->
<!-- @test('compile') -->
<p>Also, the examples below will also be written as if the namespace alias</p>
<pre class="literal-block">
namespace parameter = boost::parameter;
</pre>
<!-- @ignore() -->
<p>has been declared: we'll write <tt class="docutils literal"><span class="pre">parameter::xxx</span></tt> instead of
<tt class="docutils literal"><span class="pre">boost::parameter::xxx</span></tt>.</p>
</div>
<div class="section" id="the-abstract-interface-to-dfs">
<h3>2.1.2&nbsp;&nbsp;&nbsp;The Abstract Interface to <tt class="docutils literal">depth_first_search</tt></h3>
<p>The Graph library's <tt class="docutils literal">depth_first_search</tt> algorithm is a generic function accepting
from one to four arguments by reference.  If all arguments were
required, its signature might be as follows:</p>
<pre class="literal-block">
template &lt;
    typename Graph
  , typename DFSVisitor
  , typename Index
  , typename ColorMap
&gt;
void
    depth_first_search(
        Graph const&amp; graph
      , DFSVisitor visitor
      , typename graph_traits&lt;g&gt;::vertex_descriptor root_vertex
      , IndexMap index_map
      , ColorMap&amp; color
    );
</pre>
<!-- @ignore() -->
<p>However, most of the parameters have a useful default value,
as shown in the table below.</p>
<span id="parameter-table"></span><table border="1" class="docutils" id="default-expressions">
<caption><tt class="docutils literal">depth_first_search</tt> Parameters</caption>
<colgroup>
<col width="20%" />
<col width="7%" />
<col width="30%" />
<col width="43%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Parameter
Name</th>
<th class="head">Data
Flow</th>
<th class="head">Type</th>
<th class="head">Default Value
(if any)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal">graph</tt></td>
<td>in</td>
<td>Model of
<a class="reference external" href="../../../graph/doc/IncidenceGraph.html"><span class="concept">Incidence Graph</span></a> and
<a class="reference external" href="../../../graph/doc/VertexListGraph.html"><span class="concept">Vertex List Graph</span></a></td>
<td>none - this argument is required.</td>
</tr>
<tr><td><tt class="docutils literal">visitor</tt></td>
<td>in</td>
<td>Model of <a class="reference external" href="../../../graph/doc/DFSVisitor.html"><span class="concept">DFS Visitor</span></a></td>
<td><tt class="docutils literal"><span class="pre">boost::dfs_visitor&lt;&gt;()</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">root_vertex</tt></td>
<td>in</td>
<td><tt class="docutils literal">graph</tt>'s vertex
descriptor type.</td>
<td><tt class="docutils literal"><span class="pre">*vertices(graph).first</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">index_map</tt></td>
<td>in</td>
<td>Model of
<a class="reference external" href="../../../property_map/doc/ReadablePropertyMap.html"><span class="concept">Readable Property Map</span></a>
with key type :=
<tt class="docutils literal">graph</tt>'s vertex
descriptor and value
type an integer type.</td>
<td><tt class="docutils literal"><span class="pre">get(boost::vertex_index,graph)</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">color_map</tt></td>
<td>in /
out</td>
<td>Model of
<a class="reference external" href="../../../property_map/doc/ReadWritePropertyMap.html"><span class="concept">Read/Write Property Map</span></a>
with key type :=
<tt class="docutils literal">graph</tt>'s vertex
descriptor type.</td>
<td>a <tt class="docutils literal"><span class="pre">boost::iterator_property_map</span></tt>
created from a <tt class="docutils literal"><span class="pre">std::vector</span></tt> of
<tt class="docutils literal">default_color_type</tt> of size
<tt class="docutils literal">num_vertices(graph)</tt> and using
<tt class="docutils literal">index_map</tt> for the index map.</td>
</tr>
</tbody>
</table>
<p>Don't be intimidated by the information in the second and third columns
above.  For the purposes of this exercise, you don't need to understand
them in detail.</p>
</div>
<div class="section" id="defining-the-keywords">
<h3>2.1.3&nbsp;&nbsp;&nbsp;Defining the Keywords</h3>
<p>The point of this exercise is to make it possible to call
<tt class="docutils literal">depth_first_search</tt> with named arguments, leaving out any
arguments for which the default is appropriate:</p>
<pre class="literal-block">
graphs::depth_first_search(g, <strong>color_map_=my_color_map</strong>);
</pre>
<!-- @ignore() -->
<p>To make that syntax legal, there needs to be an object called
“<tt class="docutils literal">color_map_</tt>” whose assignment operator can accept a
<tt class="docutils literal">my_color_map</tt> argument.  In this step we'll create one such
<strong>keyword object</strong> for each parameter.  Each keyword object will be
identified by a unique <strong>keyword tag type</strong>.</p>
<!-- Revisit this

We're going to define our interface in namespace ``graphs``.  Since users
need access to the keyword objects, but not the tag types, we'll define
the keyword objects so they're accessible through ``graphs``, and we'll
hide the tag types away in a nested namespace, ``graphs::tag``.  The
library provides a convenient macro for that purpose. -->
<p>We're going to define our interface in namespace <tt class="docutils literal">graphs</tt>.  The
library provides a convenient macro for defining keyword objects:</p>
<pre class="literal-block">
#include &lt;boost/parameter/name.hpp&gt;

namespace graphs {

    BOOST_PARAMETER_NAME(graph)    // Note: no semicolon
    BOOST_PARAMETER_NAME(visitor)
    BOOST_PARAMETER_NAME(root_vertex)
    BOOST_PARAMETER_NAME(index_map)
    BOOST_PARAMETER_NAME(color_map)
}
</pre>
<!-- @test('compile') -->
<p>The declaration of the <tt class="docutils literal">graph</tt> keyword you see here is equivalent to:</p>
<pre class="literal-block">
namespace graphs {
    namespace tag {

        // keyword tag type
        struct graph
        {
            typedef boost::parameter::forward_reference qualifier;
        };
    }

    namespace // unnamed
    {
        // A reference to the keyword object
        boost::parameter::keyword&lt;tag::graph&gt; const&amp; _graph
            = boost::parameter::keyword&lt;tag::graph&gt;::instance;
    }
}
</pre>
<!-- @example.prepend('#include <boost/parameter/keyword.hpp>') -->
<!-- @test('compile') -->
<p>It defines a <em>keyword tag type</em> named <tt class="docutils literal"><span class="pre">tag::graph</span></tt> and a <em>keyword object</em>
reference named <tt class="docutils literal">_graph</tt>.</p>
<p>This “fancy dance” involving an unnamed namespace and references is all done
to avoid violating the One Definition Rule (ODR)<a class="footnote-reference" href="#odr" id="id5"><sup>2</sup></a> when the named
parameter interface is used by function templates that are instantiated in
multiple translation units (MSVC6.x users see <a class="reference internal" href="#compiler-can-t-see-references-in-unnamed-namespace">this note</a>).</p>
</div>
<div class="section" id="writing-the-function">
<h3>2.1.4&nbsp;&nbsp;&nbsp;Writing the Function</h3>
<p>Now that we have our keywords defined, the function template definition
follows a simple pattern using the <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> macro:</p>
<pre class="literal-block">
#include &lt;boost/parameter/preprocessor.hpp&gt;

namespace graphs {

    BOOST_PARAMETER_FUNCTION(
        (void),                 // 1. parenthesized return type
        depth_first_search,     // 2. name of the function template
        tag,                    // 3. namespace of tag types
        (required (graph, *) )  // 4. one required parameter, and
        (optional               //    four optional parameters,
                                //    with defaults
            (visitor,     *, boost::dfs_visitor&lt;&gt;())
            (root_vertex, *, *vertices(graph).first)
            (index_map,   *, get(boost::vertex_index,graph))
            (color_map,   *,
                default_color_map(num_vertices(graph), index_map)
            )
        )
    )
    {
        // ... body of function goes here...
        // use graph, visitor, index_map, and color_map
    }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter/name.hpp>

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(in(root_vertex))
BOOST_PARAMETER_NAME(in(index_map))
BOOST_PARAMETER_NAME(in_out(color_map))

namespace boost {

    template <typename T = int>
    struct dfs_visitor
    {
    };

    int vertex_index = 0;
}
''') -->
<!-- @test('compile') -->
<p>The arguments to <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> are:</p>
<ol class="arabic simple">
<li>The return type of the resulting function template.  Parentheses around
the return type prevent any commas it might contain from confusing the
preprocessor, and are always required.</li>
<li>The name of the resulting function template.</li>
<li>The name of a namespace where we can find tag types whose names match the
function's parameter names.</li>
<li>The function signature.</li>
</ol>
</div>
<div class="section" id="function-signatures">
<h3>2.1.5&nbsp;&nbsp;&nbsp;Function Signatures</h3>
<p>Function signatures are described as one or two adjacent parenthesized terms
(a <a class="reference external" href="../../../preprocessor/doc/index.html">Boost.Preprocessor</a> <a class="reference external" href="http://boost-consulting.com/mplbook/preprocessor.html#sequences">sequence</a>) describing the function's parameters in the
order in which they'd be expected if passed positionally.  Any required
parameters must come first, but the <tt class="docutils literal">(required … )</tt> clause can be omitted
when all the parameters are optional.</p>
<div class="section" id="required-parameters">
<h4>2.1.5.1&nbsp;&nbsp;&nbsp;Required Parameters</h4>
<div class="compound">
<p class="compound-first">Required parameters are given first—nested in a <tt class="docutils literal">(required … )</tt>
clause—as a series of two-element tuples describing each parameter name
and any requirements on the argument type.  In this case there is only a
single required parameter, so there's just a single tuple:</p>
<pre class="compound-middle literal-block">
(required <strong>(graph, *)</strong> )
</pre>
<p class="compound-last">Since <tt class="docutils literal">depth_first_search</tt> doesn't require any particular type for its
<tt class="docutils literal">graph</tt> parameter, we use an asterix to indicate that any type is
allowed.  Required parameters must always precede any optional parameters
in a signature, but if there are <em>no</em> required parameters, the
<tt class="docutils literal">(required … )</tt> clause can be omitted entirely.</p>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(graph)

BOOST_PARAMETER_FUNCTION((void), f, tag,
''') -->
<!-- @example.append(') {}') -->
<!-- @test('compile') -->
</div>
<div class="section" id="optional-parameters">
<h4>2.1.5.2&nbsp;&nbsp;&nbsp;Optional Parameters</h4>
<div class="compound">
<p class="compound-first">Optional parameters—nested in an <tt class="docutils literal">(optional … )</tt> clause—are given as a
series of adjacent <em>three</em>-element tuples describing the parameter name,
any requirements on the argument type, <em>and</em> and an expression
representing the parameter's default value:</p>
<pre class="compound-last literal-block">
(optional
    <strong>(visitor,     *, boost::dfs_visitor&lt;&gt;())
    (root_vertex, *, *vertices(graph).first)
    (index_map,   *, get(boost::vertex_index,graph))
    (color_map,   *,
        default_color_map(num_vertices(graph), index_map)
    )</strong>
)
</pre>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace boost {

    int vertex_index = 0;

    template <typename T = int>
    struct dfs_visitor
    {
    };
}

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(in(root_vertex))
BOOST_PARAMETER_NAME(in(index_map))
BOOST_PARAMETER_NAME(in_out(color_map))

BOOST_PARAMETER_FUNCTION((void), f, tag,
    (required (graph, \*))
''') -->
<!-- @example.append(') {}') -->
<!-- @test('compile') -->
</div>
<div class="section" id="handling-in-out-consume-move-from-and-forward-parameters">
<h4>2.1.5.3&nbsp;&nbsp;&nbsp;Handling “In”, “Out”, “Consume / Move-From”, and “Forward” Parameters</h4>
<div class="compound">
<p class="compound-first">By default, Boost.Parameter treats all parameters as if they were
<em>forward</em> <a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a>, which functions would take in by rvalue reference
and only <tt class="docutils literal"><span class="pre">std::forward</span></tt> or <tt class="docutils literal"><span class="pre">boost::forward</span></tt> to other functions.  Such
parameters can be <tt class="docutils literal">const</tt> lvalues, mutable lvalues, <tt class="docutils literal">const</tt> rvalues,
or mutable rvalues.  Therefore, the default configuration grants the most
flexibility to user code.  However:</p>
<ul class="compound-middle simple">
<li>Users can configure one or more parameters to be <em>in</em> <a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a>,
which can fall into the same categories as <em>forward</em> <a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a> but
are now passed by <tt class="docutils literal">const</tt> lvalue reference and so must only be read
from.  Continuing from the previous example, to indicate that
<tt class="docutils literal">root_vertex</tt> and <tt class="docutils literal">index_map</tt> are read-only, we wrap their names
in <tt class="docutils literal"><span class="pre">in(…)</span></tt>.</li>
<li>Users can configure one or more parameters to be either <em>out</em>
<a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a>, which functions would strictly write to, or <em>in-out</em>
<a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a>, which functions would both read from and write
to.  Such parameters can only be mutable lvalues.  In the example, to
indicate that <tt class="docutils literal">color_map</tt> is read-write, we wrap its name in
<tt class="docutils literal"><span class="pre">in_out(…)</span></tt>.  Note that Boost.Parameter sees no functional
difference between <tt class="docutils literal"><span class="pre">out(…)</span></tt> and <tt class="docutils literal"><span class="pre">in_out(…)</span></tt>, so you may choose
whichever makes your interfaces more self-documenting.</li>
<li>Users can configure one or more parameters to be <em>consume</em> or
<em>move-from</em> <a class="reference external" href="http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters">parameters</a>, which functions would take in by mutable
rvalue reference and <tt class="docutils literal"><span class="pre">std::move</span></tt> or <tt class="docutils literal"><span class="pre">boost::move</span></tt> as the last
access step.  Such parameters can only be mutable
rvalues.  Boost.Parameter supports wrapping the corresponding names in
<tt class="docutils literal"><span class="pre">consume(…)</span></tt> or <tt class="docutils literal"><span class="pre">move_from(…)</span></tt>.</li>
</ul>
<pre class="compound-middle literal-block">
BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(<strong>in(root_vertex)</strong>)
BOOST_PARAMETER_NAME(<strong>in(index_map)</strong>)
BOOST_PARAMETER_NAME(<strong>in_out(color_map)</strong>)
</pre>
<p class="compound-last">In order to see what happens when parameters are bound to arguments that
violate their category constraints, attempt to compile the <a class="reference external" href="../../test/compose.cpp">compose.cpp</a>
test program with either the <tt class="docutils literal">LIBS_PARAMETER_TEST_COMPILE_FAILURE_0</tt>
macro or the <tt class="docutils literal">LIBS_PARAMETER_TEST_COMPILE_FAILURE_1</tt> macro
<tt class="docutils literal">#defined</tt>.  You should encounter a compiler error caused by a specific
constraint violation.</p>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace boost {

    int vertex_index = 0;

    template <typename T = int>
    struct dfs_visitor
    {
    };
}
''') -->
<!-- @example.append('''
BOOST_PARAMETER_FUNCTION((void), f, tag,
    (required (graph, \*))
    (optional
        (visitor,     \*, boost::dfs_visitor<>())
        (root_vertex, \*, \*vertices(graph).first)
        (index_map,   \*, get(boost::vertex_index, graph))
        (color_map,   \*,
            default_color_map(num_vertices(graph), index_map)
        )
    )
)
{
}
''') -->
<!-- @test('compile') -->
</div>
<div class="section" id="positional-arguments">
<h4>2.1.5.4&nbsp;&nbsp;&nbsp;Positional Arguments</h4>
<p>When arguments are passed positionally (without the use of keywords), they
will be mapped onto parameters in the order the parameters are given in the
signature, so for example in this call</p>
<pre class="literal-block">
graphs::depth_first_search(x, y);
</pre>
<!-- @ignore() -->
<p><tt class="docutils literal">x</tt> will always be interpreted as a graph and <tt class="docutils literal">y</tt> will always be
interpreted as a visitor.</p>
</div>
<div class="section" id="default-expression-evaluation">
<h4>2.1.5.5&nbsp;&nbsp;&nbsp;Default Expression Evaluation</h4>
<div class="compound">
<p class="compound-first">Note that in our example, the value of the graph parameter is used in the
default expressions for <tt class="docutils literal">root_vertex</tt>, <tt class="docutils literal">index_map</tt>, and <tt class="docutils literal">color_map</tt>.</p>
<pre class="compound-last literal-block">
(required (<strong>graph</strong>, *) )
(optional
    (visitor,     *, boost::dfs_visitor&lt;&gt;())
    (root_vertex, *, *vertices(<strong>graph</strong>).first)
    (index_map,   *, get(boost::vertex_index, <strong>graph</strong>))
    (color_map,   *,
        default_color_map(num_vertices(<strong>graph</strong>), index_map)
    )
)
</pre>
</div>
<!-- @ignore()

A default expression is evaluated in the context of all preceding
parameters, so you can use any of their values by name. -->
<div class="compound">
<p class="compound-first">A default expression is never evaluated—or even instantiated—if an actual
argument is passed for that parameter.  We can actually demonstrate that
with our code so far by replacing the body of <tt class="docutils literal">depth_first_search</tt> with
something that prints the arguments:</p>
<pre class="compound-middle literal-block">
#include &lt;boost/graph/depth_first_search.hpp&gt;  // for dfs_visitor

BOOST_PARAMETER_FUNCTION(
    (bool), depth_first_search, tag
    <em>…signature goes here…</em>
)
{
    std::cout &lt;&lt; &quot;graph=&quot; &lt;&lt; graph;
    std::cout &lt;&lt; std::endl;
    std::cout &lt;&lt; &quot;visitor=&quot; &lt;&lt; visitor;
    std::cout &lt;&lt; std::endl;
    std::cout &lt;&lt; &quot;root_vertex=&quot; &lt;&lt; root_vertex;
    std::cout &lt;&lt; std::endl;
    std::cout &lt;&lt; &quot;index_map=&quot; &lt;&lt; index_map;
    std::cout &lt;&lt; std::endl;
    std::cout &lt;&lt; &quot;color_map=&quot; &lt;&lt; color_map;
    std::cout &lt;&lt; std::endl;
    return true;
}

#include &lt;boost/core/lightweight_test.hpp&gt;

int main()
{
    char const* g = &quot;1&quot;;
    depth_first_search(1, 2, 3, 4, 5);
    depth_first_search(
        g, '2', _color_map = '5',
        _index_map = &quot;4&quot;, _root_vertex = &quot;3&quot;
    );
    return boost::report_errors();
}
</pre>
<p class="compound-last">Despite the fact that default expressions such as
<tt class="docutils literal"><span class="pre">vertices(graph).first</span></tt> are ill-formed for the given <tt class="docutils literal">graph</tt>
arguments, both calls will compile, and each one will print exactly the
same thing.</p>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)
''') -->
<!-- @example.replace_emphasis('''
, (required
      (graph, \*)
      (visitor, \*)
      (root_vertex, \*)
      (index_map, \*)
      (color_map, \*)
  )
  ''') -->
<!-- @test('run') -->
</div>
<div class="section" id="signature-matching-and-overloading">
<h4>2.1.5.6&nbsp;&nbsp;&nbsp;Signature Matching and Overloading</h4>
<p>In fact, the function signature is so general that any call to
<tt class="docutils literal">depth_first_search</tt> with fewer than five arguments will match our function,
provided we pass <em>something</em> for the required <tt class="docutils literal">graph</tt> parameter.  That might
not seem to be a problem at first; after all, if the arguments don't match the
requirements imposed by the implementation of <tt class="docutils literal">depth_first_search</tt>, a
compilation error will occur later, when its body is instantiated.</p>
<p>There are at least three problems with very general function signatures.</p>
<ol class="arabic simple">
<li>By the time our <tt class="docutils literal">depth_first_search</tt> is instantiated, it has been
selected as the best matching overload.  Some other <tt class="docutils literal">depth_first_search</tt>
overload might've worked had it been chosen instead.  By the time we see a
compilation error, there's no chance to change that decision.</li>
<li>Even if there are no overloads, error messages generated at instantiation
time usually expose users to confusing implementation details.  For
example, users might see references to names generated by
<tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> such as
<tt class="docutils literal"><span class="pre">graphs::detail::depth_first_search_with_named_params</span></tt> (or worse—think
of the kinds of errors you get from your STL implementation when you make
a mistake).<a class="footnote-reference" href="#conceptsts" id="id7"><sup>4</sup></a></li>
<li>The problems with exposing such permissive function template signatures
have been the subject of much discussion, especially in the presence of
<a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#225">unqualified calls</a>.  If all we want is to avoid unintentional
argument-dependent lookup (ADL), we can isolate <tt class="docutils literal">depth_first_search</tt> in
a namespace containing no types<a class="footnote-reference" href="#using" id="id8"><sup>6</sup></a>, but suppose we <em>want</em> it to
found via ADL?</li>
</ol>
<p>It's usually a good idea to prevent functions from being considered for
overload resolution when the passed argument types aren't appropriate.  The
library already does this when the required <tt class="docutils literal">graph</tt> parameter is not
supplied, but we're not likely to see a depth first search that doesn't take a
graph to operate on.  Suppose, instead, that we found a different depth first
search algorithm that could work on graphs that don't model
<a class="reference external" href="../../../graph/doc/IncidenceGraph.html"><span class="concept">Incidence Graph</span></a>?  If we just added a simple overload, it would be
ambiguous:</p>
<pre class="literal-block">
// new overload
BOOST_PARAMETER_FUNCTION((void), depth_first_search, (tag),
    (required (graph,*))( … )
)
{
    // new algorithm implementation
}

…

// ambiguous!
depth_first_search(boost::adjacency_list&lt;&gt;(), 2, &quot;hello&quot;);
</pre>
<!-- @ignore() -->
<div class="section" id="predicate-requirements">
<h5>2.1.5.6.1&nbsp;&nbsp;&nbsp;Predicate Requirements</h5>
<p>We really don't want the compiler to consider the original version of
<tt class="docutils literal">depth_first_search</tt> because the <tt class="docutils literal">root_vertex</tt> argument, <tt class="docutils literal">&quot;hello&quot;</tt>,
doesn't meet the <a class="reference internal" href="#parameter-table">requirement</a> that it match the <tt class="docutils literal">graph</tt> parameter's vertex
descriptor type.  Instead, this call should just invoke our new overload.  To
take the original <tt class="docutils literal">depth_first_search</tt> overload out of contention, we first
encode this requirement as follows:</p>
<pre class="literal-block">
struct vertex_descriptor_predicate
{
    template &lt;typename T, typename Args&gt;
    struct apply
      : boost::mpl::if_&lt;
            boost::is_convertible&lt;
                T
              , typename boost::graph_traits&lt;
                    typename boost::parameter::value_type&lt;
                        Args
                      , graphs::graph
                    &gt;::type
                &gt;::vertex_descriptor
            &gt;
          , boost::mpl::true_
          , boost::mpl::false_
        &gt;
    {
    };
};
</pre>
<p>This encoding is an <a class="reference external" href="../../../mpl/doc/refmanual/metafunction-class.html">MPL Binary Metafunction Class</a>, a type with a nested
<tt class="docutils literal">apply</tt> metafunction that takes in two template arguments.  For the first
template argument, Boost.Parameter will pass in the type on which we will
impose the requirement.  For the second template argument, Boost.Parameter
will pass in the entire argument pack, making it possible to extract the
type of each of the other <tt class="docutils literal">depth_first_search</tt> parameters via the
<tt class="docutils literal">value_type</tt> metafunction and the corresponding keyword tag type.  The
result <tt class="docutils literal">type</tt> of the <tt class="docutils literal">apply</tt> metafunction will be equivalent to
<tt class="docutils literal"><span class="pre">boost::mpl::true_</span></tt> if <tt class="docutils literal">T</tt> fulfills our requirement as imposed by the
<tt class="docutils literal"><span class="pre">boost::is_convertible</span></tt> statement; otherwise, the result will be
equivalent to <tt class="docutils literal"><span class="pre">boost::mpl::false_</span></tt>.</p>
<p>At this point, we can append the name of our metafunction class, in
parentheses, to the appropriate <tt class="docutils literal">*</tt> element of the function signature.</p>
<pre class="literal-block">
(root_vertex
  , *(<strong>vertex_descriptor_predicate</strong>)
  , *vertices(graph).first
)
</pre>
<!-- @ignore() -->
<p>Now the original <tt class="docutils literal">depth_first_search</tt> will only be called when the
<tt class="docutils literal">root_vertex</tt> argument can be converted to the graph's vertex descriptor
type, and our example that <em>was</em> ambiguous will smoothly call the new
overload.</p>
<p>We can encode the requirements on other arguments using the same concept; only
the implementation of the nested <tt class="docutils literal">apply</tt> metafunction needs to be tweaked
for each argument.  There's no space to give a complete description of graph
library details here, but suffice it to show that the next few metafunction
classes provide the necessary checks.</p>
<pre class="literal-block">
struct graph_predicate
{
    template &lt;typename T, typename Args&gt;
    struct apply
      : boost::mpl::eval_if&lt;
            boost::is_convertible&lt;
                typename boost::graph_traits&lt;T&gt;::traversal_category
              , boost::incidence_graph_tag
            &gt;
          , boost::mpl::if_&lt;
                boost::is_convertible&lt;
                    typename boost::graph_traits&lt;T&gt;::traversal_category
                  , boost::vertex_list_graph_tag
                &gt;
              , boost::mpl::true_
              , boost::mpl::false_
            &gt;
        &gt;
    {
    };
};

struct index_map_predicate
{
    template &lt;typename T, typename Args&gt;
    struct apply
      : boost::mpl::eval_if&lt;
            boost::is_integral&lt;
                typename boost::property_traits&lt;T&gt;::value_type
            &gt;
          , boost::mpl::if_&lt;
                boost::is_same&lt;
                    typename boost::property_traits&lt;T&gt;::key_type
                  , typename boost::graph_traits&lt;
                        typename boost::parameter::value_type&lt;
                            Args
                          , graphs::graph
                        &gt;::type
                    &gt;::vertex_descriptor
                &gt;
              , boost::mpl::true_
              , boost::mpl::false_
            &gt;
        &gt;
    {
    };
};

struct color_map_predicate
{
    template &lt;typename T, typename Args&gt;
    struct apply
      : boost::mpl::if_&lt;
            boost::is_same&lt;
                typename boost::property_traits&lt;T&gt;::key_type
              , typename boost::graph_traits&lt;
                    typename boost::parameter::value_type&lt;
                        Args
                      , graphs::graph
                    &gt;::type
                &gt;::vertex_descriptor
            &gt;
          , boost::mpl::true_
          , boost::mpl::false_
        &gt;
    {
    };
};
</pre>
<p>Likewise, computing the default value for the <tt class="docutils literal">color_map</tt> parameter is no
trivial matter, so it's best to factor the computation out to a separate
function template.</p>
<pre class="literal-block">
template &lt;typename Size, typename IndexMap&gt;
boost::iterator_property_map&lt;
    std::vector&lt;boost::default_color_type&gt;::iterator
  , IndexMap
  , boost::default_color_type
  , boost::default_color_type&amp;
&gt;&amp;
    default_color_map(Size num_vertices, IndexMap const&amp; index_map)
{
    static std::vector&lt;boost::default_color_type&gt; colors(num_vertices);
    static boost::iterator_property_map&lt;
        std::vector&lt;boost::default_color_type&gt;::iterator
      , IndexMap
      , boost::default_color_type
      , boost::default_color_type&amp;
    &gt; m(colors.begin(), index_map);
    return m;
}
</pre>
<p>The signature encloses each predicate metafunction in parentheses <em>preceded
by an asterix</em>, as follows:</p>
<pre class="literal-block">
BOOST_PARAMETER_FUNCTION((void), depth_first_search, graphs,
(required
    (graph, *(<strong>graph_predicate</strong>))
)
(optional
    (visitor
      , *  // not easily checkable
      , boost::dfs_visitor&lt;&gt;()
    )
    (root_vertex
      , (<strong>vertex_descriptor_predicate</strong>)
      , *vertices(graph).first
    )
    (index_map
      , *(<strong>index_map_predicate</strong>)
      , get(boost::vertex_index, graph)
    )
    (color_map
      , *(<strong>color_map_predicate</strong>)
      , default_color_map(num_vertices(graph), index_map)
    )
)
)
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/depth_first_search.hpp>
#include <boost/graph/graph_traits.hpp>
#include <boost/property_map/property_map.hpp>
#include <boost/mpl/and.hpp>
#include <boost/type_traits/is_convertible.hpp>
#include <boost/type_traits/is_integral.hpp>
#include <boost/type_traits/is_same.hpp>
#include <vector>
#include <utility>

BOOST_PARAMETER_NAME((_graph, graphs) graph)
BOOST_PARAMETER_NAME((_visitor, graphs) visitor)
BOOST_PARAMETER_NAME((_root_vertex, graphs) in(root_vertex))
BOOST_PARAMETER_NAME((_index_map, graphs) in(index_map))
BOOST_PARAMETER_NAME((_color_map, graphs) in_out(color_map))
''') -->
<!-- @example.append('''
{
}

#include <boost/core/lightweight_test.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <utility>

int main()
{
    typedef boost::adjacency_list<
        boost::vecS, boost::vecS, boost::directedS
    > G;
    enum {u, v, w, x, y, z, N};
    typedef std::pair<int, int> E;
    E edges[] = {
        E(u, v), E(u, x), E(x, v), E(y, x),
        E(v, y), E(w, y), E(w,z), E(z, z)
    };
    G g(edges, edges + sizeof(edges) / sizeof(E), N);

    depth_first_search(g);
    depth_first_search(g, _root_vertex = static_cast<int>(x));
    return boost::report_errors();
}
''') -->
<!-- @test('run') -->
<p>It usually isn't necessary to so completely encode the type requirements on
arguments to generic functions.  However, doing so is worth the effort: your
code will be more self-documenting and will often provide a better user
experience.  You'll also have an easier transition to the C++20 standard with
<a class="reference internal" href="#conceptsts">language support for constraints and concepts</a>.</p>
</div>
<div class="section" id="more-on-type-requirements">
<h5>2.1.5.6.2&nbsp;&nbsp;&nbsp;More on Type Requirements</h5>
<p>Encoding type requirements onto a function's parameters is essential for
enabling the function to have deduced parameter interface.  Let's revisit the
<tt class="docutils literal">new_window</tt> example for a moment:</p>
<pre class="literal-block">
window* w = new_window(
    movable_=false
  , &quot;alert box&quot;
);
window* w = new_window(
    &quot;alert box&quot;
  , movable_=false
);
</pre>
<!-- @ignore() -->
<p>The goal this time is to be able to invoke the <tt class="docutils literal">new_window</tt> function without
specifying the keywords.  For each parameter that has a required type, we can
enclose that type in parentheses, then <em>replace</em> the <tt class="docutils literal">*</tt> element of the
parameter signature:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME((name_, keywords) name)
BOOST_PARAMETER_NAME((movable_, keywords) movable)

BOOST_PARAMETER_FUNCTION((window*), new_window, keywords,
    (deduced
        (required
            (name, <em>(char const*)</em>)
            (movable, <em>(bool)</em>)
        )
    )
)
{
    // ...
}
</pre>
<!-- @ignore() -->
<p>The following statements will now work and are equivalent to each other as
well as the previous statements:</p>
<pre class="literal-block">
window* w = new_window(false, &quot;alert box&quot;);
window* w = new_window(&quot;alert box&quot;, false);
</pre>
<!-- @ignore() -->
</div>
</div>
<div class="section" id="deduced-parameters">
<h4>2.1.5.7&nbsp;&nbsp;&nbsp;Deduced Parameters</h4>
<p>To further illustrate deduced parameter support, consider the example of the
<a class="reference external" href="../../../python/doc/v2/def.html"><tt class="docutils literal">def</tt></a> function from <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>.  Its signature is roughly as follows:</p>
<pre class="literal-block">
template &lt;
    typename Function
  , typename KeywordExpression
  , typename CallPolicies
&gt;
void def(
    // Required parameters
    char const* name, Function func

    // Optional, deduced parameters
  , char const* docstring = &quot;&quot;
  , KeywordExpression keywords = no_keywords()
  , CallPolicies policies = default_call_policies()
);
</pre>
<!-- @ignore() -->
<p>Try not to be too distracted by the use of the term “keywords” in this
example: although it means something analogous in Boost.Python to what
it means in the Parameter library, for the purposes of this exercise
you can think of it as being completely different.</p>
<p>When calling <tt class="docutils literal">def</tt>, only two arguments are required.  The association
between any additional arguments and their parameters can be determined by the
types of the arguments actually passed, so the caller is neither required to
remember argument positions or explicitly specify parameter names for those
arguments.  To generate this interface using <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt>, we
need only enclose the deduced parameters in a <tt class="docutils literal">(deduced …)</tt> clause, as
follows:</p>
<pre class="literal-block">
char const*&amp; blank_char_ptr()
{
    static char const* larr = &quot;&quot;;
    return larr;
}

BOOST_PARAMETER_FUNCTION(
    (bool), def, tag,

    (required (name, (char const*)) (func,*) )  // nondeduced

    <strong>(deduced</strong>
        (optional
            (docstring, (char const*), &quot;&quot;)

            (keywords
                // see<a class="footnote-reference" href="#is-keyword-expression" id="id13"><sup>5</sup></a>
              , *(is_keyword_expression&lt;boost::mpl::_&gt;)
              , no_keywords()
            )

            (policies
              , *(
                    boost::mpl::eval_if&lt;
                        boost::is_convertible&lt;boost::mpl::_,char const*&gt;
                      , boost::mpl::false_
                      , boost::mpl::if_&lt;
                            // see<a class="footnote-reference" href="#is-keyword-expression" id="id14"><sup>5</sup></a>
                            is_keyword_expression&lt;boost::mpl::_&gt;
                          , boost::mpl::false_
                          , boost::mpl::true_
                        &gt;
                    &gt;
                )
              , default_call_policies()
            )
        )
    <strong>)</strong>
)
{
    <em>…</em>
}
</pre>
<!-- @example.replace_emphasis('return true;') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(func)
BOOST_PARAMETER_NAME(docstring)
BOOST_PARAMETER_NAME(keywords)
BOOST_PARAMETER_NAME(policies)

struct default_call_policies
{
};

struct no_keywords
{
};

struct keywords
{
};

template <typename T>
struct is_keyword_expression
  : boost::mpl::false_
{
};

template <>
struct is_keyword_expression<keywords>
  : boost::mpl::true_
{
};

default_call_policies some_policies;

void f()
{
}

#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/type_traits/is_convertible.hpp>

''') -->
<div class="admonition admonition-syntax-note">
<p class="first admonition-title">Syntax Note</p>
<p class="last">A <tt class="docutils literal">(deduced …)</tt> clause always contains a <tt class="docutils literal">(required …)</tt> and/or an
<tt class="docutils literal">(optional …)</tt> subclause, and must follow any <tt class="docutils literal">(required …)</tt> or
<tt class="docutils literal">(optional …)</tt> clauses indicating nondeduced parameters at the outer
level.</p>
</div>
<p>With the declaration above, the following two calls are equivalent:</p>
<pre class="literal-block">
char const* f_name = &quot;f&quot;;
def(
    f_name
  , &amp;f
  , <strong>some_policies</strong>
  , <strong>&quot;Documentation for f&quot;</strong>
);
def(
    f_name
  , &amp;f
  , <strong>&quot;Documentation for f&quot;</strong>
  , <strong>some_policies</strong>
);
</pre>
<!-- @example.prepend('''
int main()
{
''') -->
<p>If the user wants to pass a <tt class="docutils literal">policies</tt> argument that was also, for some
reason, convertible to <tt class="docutils literal">char const*</tt>, she can always specify the parameter
name explicitly, as follows:</p>
<pre class="literal-block">
def(
    f_name
  , &amp;f
  , <strong>_policies = some_policies</strong>
  , &quot;Documentation for f&quot;
);
</pre>
<!-- @example.append('}') -->
<!-- @test('compile', howmany='all') -->
<p>The <a class="reference external" href="../../test/deduced.cpp">deduced.cpp</a> and <a class="reference external" href="../../test/deduced_dependent_predicate.cpp">deduced_dependent_predicate.cpp</a> test programs
demonstrate additional usage of deduced parameter support.</p>
</div>
<div class="section" id="parameter-dependent-return-types">
<h4>2.1.5.8&nbsp;&nbsp;&nbsp;Parameter-Dependent Return Types</h4>
<p>For some algorithms, the return type depends on at least one of the argument
types.  However, there is one gotcha to avoid when encoding this return type
while using <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> or other code generation macros.  As
an example, given the following definitions:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(x)
BOOST_PARAMETER_NAME(y)
BOOST_PARAMETER_NAME(z)
</pre>
<!-- @ignore() -->
<p>Let our algorithm simply return one of its arguments.  If we naïvely implement
its return type in terms of <tt class="docutils literal"><span class="pre">parameter::value_type</span></tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_FUNCTION(
    (typename parameter::value_type&lt;Args,tag::y&gt;::type), return_y, tag,
    (deduced
        (required
            (x, (std::map&lt;char const*,std::string&gt;))
            (y, (char const*))
        )
        (optional
            (z, (int), 4)
        )
    )
)
{
    return y;
}
</pre>
<!-- @ignore() -->
<p>Then using <tt class="docutils literal">return_y</tt> in any manner other than with positional arguments
will result in a compiler error:</p>
<pre class="literal-block">
std::map&lt;char const*,std::string&gt; k2s;
assert(&quot;foo&quot; == return_y(2, k2s, &quot;foo&quot;));  // error!
</pre>
<!-- @ignore() -->
<p>The problem is that even though <tt class="docutils literal">y</tt> is a required parameter,
<tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> will generate certain overloads for which the
argument pack type <tt class="docutils literal">Args</tt> does not actually contain the keyword tag type
<tt class="docutils literal"><span class="pre">tag::y</span></tt>.  The solution is to use SFINAE to preclude generation of those
overloads.  Since <tt class="docutils literal"><span class="pre">parameter::value_type</span></tt> is a metafunction, our tool for
the job is <tt class="docutils literal">lazy_enable_if</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_FUNCTION(
    (
        // Whenever using 'enable_if', 'disable_if', and so on,
        // do not add the 'typename' keyword in front.
        boost::lazy_enable_if&lt;
            typename mpl::has_key&lt;Args,tag::y&gt;::type
          , parameter::value_type&lt;Args,tag::y&gt;
        &gt;
        // Whenever using 'enable_if', 'disable_if', and so on,
        // do not add '::type' here.
    ), return_y, tag,
    (deduced
        (required
            (x, (std::map&lt;char const*,std::string&gt;))
            (y, (char const*))
        )
        (optional
            (z, (int), 4)
        )
    )
)
{
    return y;
}
</pre>
<!-- @ignore() -->
<p>For a working demonstration, see <a class="reference external" href="../../test/preprocessor_deduced.cpp">preprocessor_deduced.cpp</a>.</p>
</div>
</div>
</div>
<div class="section" id="parameter-enabled-member-functions">
<h2><a class="toc-backref" href="#id30">2.2&nbsp;&nbsp;&nbsp;Parameter-Enabled Member Functions</a></h2>
<p>The <tt class="docutils literal">BOOST_PARAMETER_MEMBER_FUNCTION</tt> and
<tt class="docutils literal">BOOST_PARAMETER_CONST_MEMBER_FUNCTION</tt> macros accept exactly the same
arguments as <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt>, but are designed to be used within
the body of a class:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)

struct callable2
{
    BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
        (void), call, tag, (required (arg1,(int))(arg2,(int)))
    )
    {
        std::cout &lt;&lt; arg1 &lt;&lt; &quot;, &quot; &lt;&lt; arg2;
        std::cout &lt;&lt; std::endl;
    }
};

#include &lt;boost/core/lightweight_test.hpp&gt;

int main()
{
    callable2 c2;
    callable2 const&amp; c2_const = c2;
    c2_const.call(1, 2);
    return boost::report_errors();
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
using namespace boost::parameter;
''') -->
<!-- @test('run') -->
<p>These macros don't directly allow a function's interface to be separated from
its implementation, but you can always forward arguments on to a separate
implementation function:</p>
<pre class="literal-block">
struct callable2
{
    BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
        (void), call, tag, (required (arg1,(int))(arg2,(int)))
    )
    {
        call_impl(arg1, arg2);
    }

 private:
    void call_impl(int, int);  // implemented elsewhere.
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)
using namespace boost::parameter;
''') -->
<!-- @test('compile') -->
<div class="section" id="static-member-functions">
<h3>2.2.1&nbsp;&nbsp;&nbsp;Static Member Functions</h3>
<p>To expose a static member function, simply insert the keyword “<tt class="docutils literal">static</tt>”
before the function name:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(arg1)

struct somebody
{
    BOOST_PARAMETER_MEMBER_FUNCTION(
        (void), <strong>static</strong> f, tag, (optional (arg1,(int),0))
    )
    {
        std::cout &lt;&lt; arg1 &lt;&lt; std::endl;
    }
};

#include &lt;boost/core/lightweight_test.hpp&gt;

int main()
{
    somebody::f();
    somebody::f(4);
    return boost::report_errors();
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
using namespace boost::parameter;
''') -->
<!-- @test('run') -->
</div>
</div>
<div class="section" id="parameter-enabled-function-call-operators">
<h2><a class="toc-backref" href="#id31">2.3&nbsp;&nbsp;&nbsp;Parameter-Enabled Function Call Operators</a></h2>
<p>The <tt class="docutils literal">BOOST_PARAMETER_FUNCTION_CALL_OPERATOR</tt> and
<tt class="docutils literal">BOOST_PARAMETER_CONST_FUNCTION_CALL_OPERATOR</tt> macros accept the same
arguments as the <tt class="docutils literal">BOOST_PARAMETER_MEMBER_FUNCTION</tt> and
<tt class="docutils literal">BOOST_PARAMETER_CONST_MEMBER_FUNCTION</tt> macros except for the function name,
because these macros allow instances of the enclosing classes to be treated as
function objects:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(first_arg)
BOOST_PARAMETER_NAME(second_arg)

struct callable2
{
    BOOST_PARAMETER_CONST_FUNCTION_CALL_OPERATOR(
        (void), tag, (required (first_arg,(int))(second_arg,(int)))
    )
    {
        std::cout &lt;&lt; first_arg &lt;&lt; &quot;, &quot;;
        std::cout &lt;&lt; second_arg &lt;&lt; std::endl;
    }
};

#include &lt;boost/core/lightweight_test.hpp&gt;

int main()
{
    callable2 c2;
    callable2 const&amp; c2_const = c2;
    c2_const(1, 2);
    return boost::report_errors();
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
using namespace boost::parameter;
''') -->
<!-- @test('run') -->
</div>
<div class="section" id="parameter-enabled-constructors">
<h2><a class="toc-backref" href="#id32">2.4&nbsp;&nbsp;&nbsp;Parameter-Enabled Constructors</a></h2>
<p>The lack of a “delegating constructor” feature in C++
(<a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf</a>)
limits somewhat the quality of interface this library can provide
for defining parameter-enabled constructors.  The usual workaround
for a lack of constructor delegation applies: one must factor the
common logic into one or more base classes.</p>
<p>Let's build a parameter-enabled constructor that simply prints its
arguments.  The first step is to write a base class whose
constructor accepts a single argument known as an <a class="reference external" href="reference.html#argumentpack"><span class="concept">ArgumentPack</span></a>:
a bundle of references to the actual arguments, tagged with their
keywords.  The values of the actual arguments are extracted from
the <span class="concept">ArgumentPack</span> by <em>indexing</em> it with keyword objects:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)

struct myclass_impl
{
    template &lt;typename ArgumentPack&gt;
    myclass_impl(ArgumentPack const&amp; args)
    {
        std::cout &lt;&lt; &quot;name = &quot; &lt;&lt; args[_name];
        std::cout &lt;&lt; &quot;; index = &quot; &lt;&lt; args[_index | 42];
        std::cout &lt;&lt; std::endl;
    }
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
''') -->
<p>Note that the bitwise or (“<tt class="docutils literal">|</tt>”) operator has a special meaning when
applied to keyword objects that are passed to an <span class="concept">ArgumentPack</span>'s indexing
operator: it is used to indicate a default value.  In this case if there is no
<tt class="docutils literal">index</tt> parameter in the <span class="concept">ArgumentPack</span>, <tt class="docutils literal">42</tt> will be used instead.</p>
<p>Now we are ready to write the parameter-enabled constructor interface:</p>
<pre class="literal-block">
struct myclass : myclass_impl
{
    BOOST_PARAMETER_CONSTRUCTOR(
        myclass, (myclass_impl), tag
      , (required (name,*)) (optional (index,*))
    ) // no semicolon
};
</pre>
<p>Since we have supplied a default value for <tt class="docutils literal">index</tt> but not for <tt class="docutils literal">name</tt>,
only <tt class="docutils literal">name</tt> is required.  We can exercise our new interface as follows:</p>
<pre class="literal-block">
myclass x(&quot;bob&quot;, 3);                      // positional
myclass y(_index = 12, _name = &quot;sally&quot;);  // named
myclass z(&quot;june&quot;);                        // positional/defaulted
</pre>
<!-- @example.wrap('''
#include <boost/core/lightweight_test.hpp>

int main() {
''', ' return boost::report_errors(); }') -->
<!-- @test('run', howmany='all') -->
<p>For more on <span class="concept">ArgumentPack</span> manipulation, see the <a class="reference internal" href="#advanced-topics">Advanced Topics</a> section.</p>
</div>
<div class="section" id="parameter-enabled-class-templates">
<h2><a class="toc-backref" href="#id33">2.5&nbsp;&nbsp;&nbsp;Parameter-Enabled Class Templates</a></h2>
<p>In this section we'll use Boost.Parameter to build <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>'s <a class="reference external" href="http://www.boost.org/libs/python/doc/v2/class.html#class_-spec">class_</a> template, whose “signature” is:</p>
<pre class="literal-block">
template &lt;
    ValueType, BaseList = bases&lt;&gt;
  , HeldType = ValueType, Copyable = void
&gt;
class class_;
</pre>
<!-- @ignore() -->
<p>Only the first argument, <tt class="docutils literal">ValueType</tt>, is required.</p>
<div class="section" id="named-template-parameters">
<h3>2.5.1&nbsp;&nbsp;&nbsp;Named Template Parameters</h3>
<p>First, we'll build an interface that allows users to pass arguments
positionally or by name:</p>
<pre class="literal-block">
struct B
{
    virtual ~B() = 0;
};

struct D : B
{
    ~D();
};

class_&lt;
    <strong>class_type&lt;B&gt;</strong>
  , <strong>copyable&lt;boost::noncopyable&gt;</strong>
&gt; …;

class_&lt;
    <strong>D</strong>
  , <strong>held_type&lt;std::auto_ptr&lt;D&gt; &gt;</strong>
  , <strong>base_list&lt;bases&lt;B&gt; &gt;</strong>
&gt; …;
</pre>
<!-- @ignore() -->
<div class="section" id="template-keywords">
<h4>2.5.1.1&nbsp;&nbsp;&nbsp;Template Keywords</h4>
<p>The first step is to define keywords for each template parameter:</p>
<pre class="literal-block">
namespace boost { namespace python {

    BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)
}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @test('compile') -->
<p>The declaration of the <tt class="docutils literal">class_type</tt> keyword you see here is equivalent to:</p>
<pre class="literal-block">
namespace boost { namespace python {
    namespace tag {

        struct class_type;  // keyword tag type
    }

    template &lt;typename T&gt;
    struct class_type
      : parameter::template_keyword&lt;tag::class_type,T&gt;
    {
    };
}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @test('compile') -->
<p>It defines a keyword tag type named <tt class="docutils literal"><span class="pre">tag::class_type</span></tt> and a
<em>parameter passing template</em> named <tt class="docutils literal">class_type</tt>.</p>
</div>
<div class="section" id="class-template-skeleton">
<h4>2.5.1.2&nbsp;&nbsp;&nbsp;Class Template Skeleton</h4>
<p>The next step is to define the skeleton of our class template, which has three
optional parameters.  Because the user may pass arguments in any order, we
don't know the actual identities of these parameters, so it would be premature
to use descriptive names or write out the actual default values for any of
them.  Instead, we'll give them generic names and use the special type
<tt class="docutils literal"><span class="pre">boost::parameter::void_</span></tt> as a default:</p>
<pre class="literal-block">
namespace boost { namespace python {

    template &lt;
        typename A0
      , typename A1 = boost::parameter::void_
      , typename A2 = boost::parameter::void_
      , typename A3 = boost::parameter::void_
    &gt;
    struct class_
    {
        <em>…</em>
    };
}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @example.replace_emphasis('') -->
<!-- @test('compile') -->
</div>
<div class="section" id="class-template-signatures">
<h4>2.5.1.3&nbsp;&nbsp;&nbsp;Class Template Signatures</h4>
<p>Next, we need to build a type, known as a <a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a>, describing the
“signature” of <tt class="docutils literal"><span class="pre">boost::python::class_</span></tt>.  A <a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a> enumerates the
required and optional parameters in their positional order, along with any
type requirements (note that it does <em>not</em> specify defaults -- those will be
dealt with separately):</p>
<pre class="literal-block">
namespace boost { namespace python {

    using boost::mpl::_;

    typedef parameter::parameters&lt;
        required&lt;tag::class_type, boost::is_class&lt;_&gt; &gt;
      , parameter::optional&lt;tag::base_list, mpl::is_sequence&lt;_&gt; &gt;
      , parameter::optional&lt;tag::held_type&gt;
      , parameter::optional&lt;tag::copyable&gt;
    &gt; class_signature;
}}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/mpl/is_sequence.hpp>
#include <boost/noncopyable.hpp>
#include <boost/type_traits/is_class.hpp>
#include <memory>

using namespace boost::parameter;

namespace boost { namespace python {

    BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)

    template <typename B = int>
    struct bases
    {
    };
}}
''') -->
</div>
<div class="section" id="argument-packs-and-parameter-extraction">
<span id="binding-intro"></span><h4>2.5.1.4&nbsp;&nbsp;&nbsp;Argument Packs and Parameter Extraction</h4>
<p>Next, within the body of <tt class="docutils literal">class_</tt> , we use the <span class="concept">ParameterSpec</span>'s
nested <tt class="docutils literal">::bind&lt; … &gt;</tt> template to bundle the actual arguments into an
<a class="reference external" href="reference.html#argumentpack"><span class="concept">ArgumentPack</span></a> type, and then use the library's <tt class="docutils literal">value_type&lt; … &gt;</tt>
metafunction to extract “logical parameters”.  <tt class="docutils literal">value_type&lt; … &gt;</tt> is
a lot like <tt class="docutils literal">binding&lt; … &gt;</tt>, but no reference is added to the actual
argument type.  Note that defaults are specified by passing it an
optional third argument:</p>
<pre class="literal-block">
namespace boost { namespace python {

    template &lt;
        typename A0
      , typename A1 = boost::parameter::void_
      , typename A2 = boost::parameter::void_
      , typename A3 = boost::parameter::void_
    &gt;
    struct class_
    {
        // Create ArgumentPack
        typedef typename class_signature::template bind&lt;
            A0, A1, A2, A3
        &gt;::type args;

        // Extract first logical parameter.
        typedef typename parameter::value_type&lt;
            args, tag::class_type
        &gt;::type class_type;

        typedef typename parameter::value_type&lt;
            args, tag::base_list, bases&lt;&gt;
        &gt;::type base_list;

        typedef typename parameter::value_type&lt;
            args, tag::held_type, class_type
        &gt;::type held_type;

        typedef typename parameter::value_type&lt;
            args, tag::copyable, void
        &gt;::type copyable;
    };
}}
</pre>
</div>
</div>
<div class="section" id="exercising-the-code-so-far">
<h3>2.5.2&nbsp;&nbsp;&nbsp;Exercising the Code So Far</h3>
<div class="compound">
<p class="compound-first">Revisiting our original examples,</p>
<pre class="compound-middle literal-block">
typedef boost::python::class_&lt;
    class_type&lt;B&gt;, copyable&lt;boost::noncopyable&gt;
&gt; c1;

typedef boost::python::class_&lt;
    D
  , held_type&lt;std::auto_ptr&lt;D&gt; &gt;
  , base_list&lt;bases&lt;B&gt; &gt;
&gt; c2;
</pre>
<!-- @example.prepend('''
using boost::python::class_type;
using boost::python::copyable;
using boost::python::held_type;
using boost::python::base_list;
using boost::python::bases;

struct B
{
};

struct D
{
};
''') -->
<p class="compound-middle">we can now examine the intended parameters:</p>
<pre class="compound-last literal-block">
BOOST_MPL_ASSERT((boost::is_same&lt;c1::class_type, B&gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c1::base_list, bases&lt;&gt; &gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c1::held_type, B&gt;));
BOOST_MPL_ASSERT((
    boost::is_same&lt;c1::copyable, boost::noncopyable&gt;
));

BOOST_MPL_ASSERT((boost::is_same&lt;c2::class_type, D&gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c2::base_list, bases&lt;B&gt; &gt;));
BOOST_MPL_ASSERT((
    boost::is_same&lt;c2::held_type, std::auto_ptr&lt;D&gt; &gt;
));
BOOST_MPL_ASSERT((boost::is_same&lt;c2::copyable, void&gt;));
</pre>
</div>
<!-- @test('compile', howmany='all') -->
</div>
<div class="section" id="deduced-template-parameters">
<h3>2.5.3&nbsp;&nbsp;&nbsp;Deduced Template Parameters</h3>
<p>To apply a deduced parameter interface here, we need only make the type
requirements a bit tighter so the <tt class="docutils literal">held_type</tt> and <tt class="docutils literal">copyable</tt> parameters
can be crisply distinguished from the others.  <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a> does this by
requiring that <tt class="docutils literal">base_list</tt> be a specialization of its <tt class="docutils literal">bases&lt; … &gt;</tt>
template (as opposed to being any old MPL sequence) and by requiring that
<tt class="docutils literal">copyable</tt>, if explicitly supplied, be <tt class="docutils literal"><span class="pre">boost::noncopyable</span></tt>.  One easy way
of identifying specializations of <tt class="docutils literal">bases&lt; … &gt;</tt> is to derive them all from
the same class, as an implementation detail:</p>
<pre class="literal-block">
namespace boost { namespace python {
    namespace detail {

        struct bases_base
        {
        };
    }

    template &lt;
        typename A0 = void, typename A1 = void, typename A2 = void <em>…</em>
    &gt;
    struct bases <strong>: detail::bases_base</strong>
    {
    };
}}
</pre>
<!-- @example.replace_emphasis('') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/mpl/is_sequence.hpp>
#include <boost/noncopyable.hpp>
#include <memory>

using namespace boost::parameter;
using boost::mpl::_;

namespace boost { namespace python {

    BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
    BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)
}}
''') -->
<p>Now we can rewrite our signature to make all three optional parameters
deducible:</p>
<pre class="literal-block">
typedef parameter::parameters&lt;
    required&lt;tag::class_type, is_class&lt;_&gt; &gt;

  , parameter::optional&lt;
        deduced&lt;tag::base_list&gt;
      , is_base_and_derived&lt;detail::bases_base,_&gt;
    &gt;

  , parameter::optional&lt;
        deduced&lt;tag::held_type&gt;
      , mpl::not_&lt;
            mpl::or_&lt;
                is_base_and_derived&lt;detail::bases_base,_&gt;
              , is_same&lt;noncopyable,_&gt;
            &gt;
        &gt;
    &gt;

  , parameter::optional&lt;
        deduced&lt;tag::copyable&gt;
      , is_same&lt;noncopyable,_&gt;
    &gt;

&gt; class_signature;
</pre>
<!-- @example.prepend('''
#include <boost/type_traits/is_class.hpp>
namespace boost { namespace python {
''') -->
<!-- @example.append('''
    template <
        typename A0
      , typename A1 = boost::parameter::void_
      , typename A2 = boost::parameter::void_
      , typename A3 = boost::parameter::void_
    >
    struct class_
    {
        // Create ArgumentPack
        typedef typename class_signature::bind<
            A0, A1, A2, A3
        >::type args;

        // Extract first logical parameter.
        typedef typename parameter::value_type<
            args, tag::class_type
        >::type class_type;

        typedef typename parameter::value_type<
            args, tag::base_list, bases<>
        >::type base_list;

        typedef typename parameter::value_type<
            args, tag::held_type, class_type
        >::type held_type;

        typedef typename parameter::value_type<
            args, tag::copyable, void
        >::type copyable;
    };
}}
''') -->
<p>It may seem like we've added a great deal of complexity, but the benefits to
our users are greater.  Our original examples can now be written without
explicit parameter names:</p>
<pre class="literal-block">
typedef boost::python::class_&lt;<strong>B</strong>, <strong>boost::noncopyable</strong>&gt; c1;

typedef boost::python::class_&lt;
    <strong>D</strong>, <strong>std::auto_ptr&lt;D&gt;</strong>, <strong>bases&lt;B&gt;</strong>
&gt; c2;
</pre>
<!-- @example.prepend('''
struct B
{
};

struct D
{
};

using boost::python::bases;
''') -->
<!-- @example.append('''
BOOST_MPL_ASSERT((boost::is_same<c1::class_type, B>));
BOOST_MPL_ASSERT((boost::is_same<c1::base_list, bases<> >));
BOOST_MPL_ASSERT((boost::is_same<c1::held_type, B>));
BOOST_MPL_ASSERT((
    boost::is_same<c1::copyable, boost::noncopyable>
));

BOOST_MPL_ASSERT((boost::is_same<c2::class_type, D>));
BOOST_MPL_ASSERT((boost::is_same<c2::base_list, bases<B> >));
BOOST_MPL_ASSERT((
    boost::is_same<c2::held_type, std::auto_ptr<D> >
));
BOOST_MPL_ASSERT((boost::is_same<c2::copyable, void>));
''') -->
<!-- @test('compile', howmany='all') -->
</div>
</div>
</div>
<div class="section" id="advanced-topics">
<h1><a class="toc-backref" href="#id34">3&nbsp;&nbsp;&nbsp;Advanced Topics</a></h1>
<p>At this point, you should have a good grasp of the basics.  In this section
we'll cover some more esoteric uses of the library.</p>
<div class="section" id="fine-grained-name-control">
<h2><a class="toc-backref" href="#id35">3.1&nbsp;&nbsp;&nbsp;Fine-Grained Name Control</a></h2>
<p>If you don't like the leading-underscore naming convention used to refer to
keyword objects, or you need the name <tt class="docutils literal">tag</tt> for something other than the
keyword type namespace, there's another way to use <tt class="docutils literal">BOOST_PARAMETER_NAME</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(
    <strong>(</strong>
        <em>object-name</em>
      <strong>,</strong> <em>tag-namespace</em>
    <strong>)</strong> <em>parameter-name</em>
)
</pre>
<!-- @ignore() -->
<p>Here is a usage example:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(
    (
        <strong>pass_foo</strong>, <strong>keywords</strong>
    ) <strong>foo</strong>
)

BOOST_PARAMETER_FUNCTION(
    (int), f,
    <strong>keywords</strong>, (required (<strong>foo</strong>, *))
)
{
    return <strong>foo</strong> + 1;
}

int x = f(<strong>pass_foo</strong> = 41);
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @example.append('''
int main()
{
    return 0;
}
''') -->
<!-- @test('run') -->
<p>Before you use this more verbose form, however, please read the section on
<a class="reference internal" href="#keyword-naming">best practices for keyword object naming</a>.</p>
</div>
<div class="section" id="more-argumentpacks">
<h2><a class="toc-backref" href="#id36">3.2&nbsp;&nbsp;&nbsp;More <span class="concept">ArgumentPack</span>s</a></h2>
<p>We've already seen <span class="concept">ArgumentPack</span>s when we looked at
<a class="reference internal" href="#parameter-enabled-constructors">parameter-enabled constructors</a> and <a class="reference internal" href="#binding-intro">class templates</a>.  As you
might have guessed, <span class="concept">ArgumentPack</span>s actually lie at the heart of
everything this library does; in this section we'll examine ways to
build and manipulate them more effectively.</p>
<div class="section" id="building-argumentpacks">
<h3>3.2.1&nbsp;&nbsp;&nbsp;Building <span class="concept">ArgumentPack</span>s</h3>
<p>The simplest <span class="concept">ArgumentPack</span> is the result of assigning into a keyword object:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(index)

template &lt;typename ArgumentPack&gt;
int print_index(ArgumentPack const&amp; args)
{
    std::cout &lt;&lt; &quot;index = &quot; &lt;&lt; args[_index];
    std::cout &lt;&lt; std::endl;
    return 0;
}

int x = print_index(_index = 3);  // prints &quot;index = 3&quot;
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
''') -->
<p>Also, <span class="concept">ArgumentPack</span>s can be composed using the comma operator.  The extra
parentheses below are used to prevent the compiler from seeing two separate
arguments to <tt class="docutils literal">print_name_and_index</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)

template &lt;typename ArgumentPack&gt;
int print_name_and_index(ArgumentPack const&amp; args)
{
    std::cout &lt;&lt; &quot;name = &quot; &lt;&lt; args[_name];
    std::cout &lt;&lt; &quot;; &quot;;
    return print_index(args);
}

int y = print_name_and_index((_index = 3, _name = &quot;jones&quot;));
</pre>
<p>The <a class="reference external" href="../../test/compose.cpp">compose.cpp</a> test program shows more examples of this feature.</p>
<p>To build an <span class="concept">ArgumentPack</span> with positional arguments, we can use a
<a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a>.  As introduced described in the section on <a class="reference internal" href="#class-template-signatures">Class Template
Signatures</a>, a <span class="concept">ParameterSpec</span> describes the positional order of parameters
and any associated type requirements.  Just as we can build an <span class="concept">ArgumentPack</span>
<em>type</em> with its nested <tt class="docutils literal">::bind&lt; … &gt;</tt> template, we can build an
<span class="concept">ArgumentPack</span> <em>object</em> by invoking its function call operator:</p>
<pre class="literal-block">
parameter::parameters&lt;
    required&lt;tag::name, is_convertible&lt;_,char const*&gt; &gt;
  , optional&lt;tag::index, is_convertible&lt;_,int&gt; &gt;
&gt; spec;

char const sam[] = &quot;sam&quot;;
int twelve = 12;

int z0 = print_name_and_index(
    <strong>spec(</strong> sam, twelve <strong>)</strong>
);

int z1 = print_name_and_index(
    <strong>spec(</strong> _index=12, _name=&quot;sam&quot; <strong>)</strong>
);
</pre>
<!-- @example.prepend('''
namespace parameter = boost::parameter;
using parameter::required;
using parameter::optional;
using boost::is_convertible;
using boost::mpl::_;
''') -->
<!-- @example.append('''
int main()
{
    return 0;
}
''') -->
<!-- @test('run', howmany='all') -->
</div>
<div class="section" id="extracting-parameter-types">
<h3>3.2.2&nbsp;&nbsp;&nbsp;Extracting Parameter Types</h3>
<p>If we want to know the types of the arguments passed to
<tt class="docutils literal">print_name_and_index</tt>, we have a couple of options.  The
simplest and least error-prone approach is to forward them to a
function template and allow <em>it</em> to do type deduction:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)

template &lt;typename Name, typename Index&gt;
int deduce_arg_types_impl(Name&amp;&amp; name, Index&amp;&amp; index)
{
    // we know the types
    Name&amp;&amp; n2 = boost::forward&lt;Name&gt;(name);
    Index&amp;&amp; i2 = boost::forward&lt;Index&gt;(index);
    return index;
}

template &lt;typename ArgumentPack&gt;
int deduce_arg_types(ArgumentPack const&amp; args)
{
    return deduce_arg_types_impl(args[_name], args[_index | 42]);
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
''') -->
<!-- @example.append('''
#include <boost/core/lightweight_test.hpp>

int main()
{
    int a1 = deduce_arg_types((_name = "foo"));
    int a2 = deduce_arg_types((_name = "foo", _index = 3));
    BOOST_TEST_EQ(a1, 42);
    BOOST_TEST_EQ(a2, 3);
    return boost::report_errors();
}
''') -->
<!-- @test('run') -->
<p>Occasionally one needs to deduce argument types without an extra layer of
function call.  For example, suppose we wanted to return twice the value of
the <tt class="docutils literal">index</tt> parameter?  In that case we can use the <tt class="docutils literal">value_type&lt; … &gt;</tt>
metafunction introduced <a class="reference internal" href="#binding-intro">earlier</a>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(index)

template &lt;typename ArgumentPack&gt;
typename boost::parameter::value_type&lt;ArgumentPack,tag::index,int&gt;::type
    twice_index(ArgumentPack const&amp; args)
{
    return 2 * args[_index | 42];
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
''') -->
<!-- @example.append('''
#include <boost/core/lightweight_test.hpp>

int main()
{
    int six = twice_index(_index = 3);
    BOOST_TEST_EQ(six, 6);
    return boost::report_errors();
}
''') -->
<!-- @test('run', howmany='all') -->
<p>Note that if we had used <tt class="docutils literal">binding&lt; … &gt;</tt> rather than <tt class="docutils literal">value_type&lt; … &gt;</tt>, we
would end up returning a reference to the temporary created in the <tt class="docutils literal">2 * …</tt>
expression.</p>
</div>
<div class="section" id="lazy-default-computation">
<h3>3.2.3&nbsp;&nbsp;&nbsp;Lazy Default Computation</h3>
<p>When a default value is expensive to compute, it would be preferable to avoid
it until we're sure it's absolutely necessary.  <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt>
takes care of that problem for us, but when using <span class="concept">ArgumentPack</span>s
explicitly, we need a tool other than <tt class="docutils literal">operator|</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template &lt;typename ArgumentPack&gt;
std::string f(ArgumentPack const&amp; args)
{
    std::string const&amp; s1 = args[_s1];
    std::string const&amp; s2 = args[_s2];
    typename parameter::binding&lt;
        ArgumentPack,tag::s3,std::string
    &gt;::type s3 = args[_s3 | (s1 + s2)];  // always constructs s1 + s2
    return s3;
}

std::string x = f((
    _s1=&quot;hello,&quot;, _s2=&quot; world&quot;, _s3=&quot;hi world&quot;
));
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <string>

namespace parameter = boost::parameter;
''') -->
<!-- @example.append('''
int main()
{
    return 0;
}
''') -->
<!-- @test('run') -->
<p>In the example above, the string <tt class="docutils literal">&quot;hello, world&quot;</tt> is constructed despite the
fact that the user passed us a value for <tt class="docutils literal">s3</tt>.  To remedy that, we can
compute the default value <em>lazily</em> (that is, only on demand), by using
<tt class="docutils literal"><span class="pre">boost::bind()</span></tt> to create a function object.</p>
<!-- danielw: I'm leaving the text below in the source, because we might -->
<!-- want to change back to it after 1.34, and if I remove it now we -->
<!-- might forget about it. -->
<!-- by combining the logical-or (“``||``”) operator -->
<!-- with a function object built by the Boost Lambda_ library: [#bind]_ -->
<pre class="literal-block">
typename parameter::binding&lt;
    ArgumentPack,tag::s3,std::string
&gt;::type s3 = args[
    _s3 <strong>|| boost::bind(
        std::plus&lt;std::string&gt;(), boost::ref(s1), boost::ref(s2)
    )</strong>
];
</pre>
<!-- @example.prepend('''
#include <boost/bind.hpp>
#include <boost/ref.hpp>
#include <boost/parameter.hpp>
#include <string>
#include <functional>

namespace parameter = boost::parameter;

BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template <typename ArgumentPack>
std::string f(ArgumentPack const& args)
{
    std::string const& s1 = args[_s1];
    std::string const& s2 = args[_s2];
''') -->
<!-- @example.append('''
    return s3;
}

std::string x = f((_s1="hello,", _s2=" world", _s3="hi world"));

int main()
{
    return 0;
}
''') -->
<!-- @test('run') -->
<!-- .. _Lambda: ../../../lambda/index.html -->
<div class="sidebar">
<p class="first sidebar-title">Mnemonics</p>
<p class="last">To remember the difference between <tt class="docutils literal">|</tt> and <tt class="docutils literal">||</tt>, recall that <tt class="docutils literal">||</tt>
normally uses short-circuit evaluation: its second argument is only
evaluated if its first argument is <tt class="docutils literal">false</tt>.  Similarly, in
<tt class="docutils literal">color_map[param || f]</tt>, <tt class="docutils literal">f</tt> is only invoked if no <tt class="docutils literal">color_map</tt>
argument was supplied.</p>
</div>
<p>The expression <tt class="docutils literal"><span class="pre">bind(std::plus&lt;std::string&gt;(),</span> ref(s1), ref(s2))</tt> yields a
<em>function object</em> that, when invoked, adds the two strings together.  That
function will only be invoked if no <tt class="docutils literal">s3</tt> argument is supplied by the caller.</p>
<!-- The expression ``lambda::var(s1) + lambda::var(s2)`` yields a -->
<!-- *function object* that, when invoked, adds the two strings -->
<!-- together.  That function will only be invoked if no ``s3`` argument -->
<!-- is supplied by the caller. -->
</div>
</div>
</div>
<div class="section" id="best-practices">
<h1><a class="toc-backref" href="#id37">4&nbsp;&nbsp;&nbsp;Best Practices</a></h1>
<p>By now you should have a fairly good idea of how to use the Parameter
library.  This section points out a few more-marginal issues that will help
you use the library more effectively.</p>
<div class="section" id="keyword-naming">
<h2><a class="toc-backref" href="#id38">4.1&nbsp;&nbsp;&nbsp;Keyword Naming</a></h2>
<p><tt class="docutils literal">BOOST_PARAMETER_NAME</tt> prepends a leading underscore to the names of all our
keyword objects in order to avoid the following usually-silent bug:</p>
<pre class="literal-block">
namespace people
{
    namespace tag
    {
        struct name
        {
            typedef boost::parameter::forward_reference qualifier;
        };

        struct age
        {
            typedef boost::parameter::forward_reference qualifier;
        };
    }

    namespace // unnamed
    {
        boost::parameter::keyword&lt;tag::name&gt;&amp; <strong>name</strong>
            = boost::parameter::keyword&lt;tag::name&gt;::instance;
        boost::parameter::keyword&lt;tag::age&gt;&amp; <strong>age</strong>
            = boost::parameter::keyword&lt;tag::age&gt;::instance;
    }

    BOOST_PARAMETER_FUNCTION(
        (void), g, tag, (optional (name, *, &quot;bob&quot;)(age, *, 42))
    )
    {
        std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; age;
    }

    void f(int age)
    {
        :vellipsis:<a href="#id18"><span class="problematic" id="id19">`</span></a>        .
        .
        .
        `
        g(<strong>age</strong> = 3);  // whoops!
    }
}
</pre>
<div class="system-message" id="id18">
<p class="system-message-title">System Message: WARNING/2 (<tt class="docutils">/root/project/libs/parameter/doc/index.rst</tt>, line 2472); <em><a href="#id19">backlink</a></em></p>
Inline interpreted text or phrase reference start-string without end-string.</div>
<!-- @ignore() -->
<p>Although in the case above, the user was trying to pass the value <tt class="docutils literal">3</tt> as the
<tt class="docutils literal">age</tt> parameter to <tt class="docutils literal">g</tt>, what happened instead was that <tt class="docutils literal">f</tt>'s <tt class="docutils literal">age</tt>
argument got reassigned the value 3, and was then passed as a positional
argument to <tt class="docutils literal">g</tt>.  Since <tt class="docutils literal">g</tt>'s first positional parameter is <tt class="docutils literal">name</tt>, the
default value for <tt class="docutils literal">age</tt> is used, and g prints <tt class="docutils literal">3:42</tt>.  Our leading
underscore naming convention makes this problem less likely to occur.</p>
<p>In this particular case, the problem could have been detected if f's <tt class="docutils literal">age</tt>
parameter had been made <tt class="docutils literal">const</tt>, which is always a good idea whenever
possible.  Finally, we recommend that you use an enclosing namespace for all
your code, but particularly for names with leading underscores.  If we were to
leave out the <tt class="docutils literal">people</tt> namespace above, names in the global namespace
beginning with leading underscores—which are reserved to your C++
compiler—might become irretrievably ambiguous with those in our
unnamed namespace.</p>
</div>
<div class="section" id="namespaces">
<h2><a class="toc-backref" href="#id39">4.2&nbsp;&nbsp;&nbsp;Namespaces</a></h2>
<p>In our examples we've always declared keyword objects in (an unnamed namespace
within) the same namespace as the Boost.Parameter-enabled functions using
those keywords:</p>
<pre class="literal-block">
namespace lib {

    <strong>BOOST_PARAMETER_NAME(name)
    BOOST_PARAMETER_NAME(index)</strong>

    BOOST_PARAMETER_FUNCTION(
        (int), f, tag,
        (optional (name,*,&quot;bob&quot;)(index,(int),1))
    )
    {
        std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; index;
        std::cout &lt;&lt; std::endl;
        return index;
    }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
''') -->
<!-- @namespace_setup = str(example) -->
<!-- @ignore() -->
<p>Users of these functions have a few choices:</p>
<ol class="arabic">
<li><p class="first">Full qualification:</p>
<pre class="literal-block">
int x = <strong>lib::</strong>f(
    <strong>lib::</strong>_name = &quot;jill&quot;
  , <strong>lib::</strong>_index = 1
);
</pre>
<p>This approach is more verbose than many users would like.</p>
</li>
</ol>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() { return 0; }') -->
<!-- @test('run') -->
<ol class="arabic" start="2">
<li><p class="first">Make keyword objects available through <em>using-declarations</em>:</p>
<pre class="literal-block">
<strong>using lib::_name;
using lib::_index;</strong>

int x = lib::f(_name = &quot;jill&quot;, _index = 1);
</pre>
<p>This version is much better at the actual call site, but the
<em>using-declarations</em> themselves can be verbose and hard to manage.</p>
</li>
</ol>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() { return 0; }') -->
<!-- @test('run') -->
<ol class="arabic" start="3">
<li><p class="first">Bring in the entire namespace with a <em>using-directive</em>:</p>
<pre class="literal-block">
<strong>using namespace lib;</strong>
int x = <strong>f</strong>(_name = &quot;jill&quot;, _index = 3);
</pre>
<p>This option is convenient, but it indiscriminately makes the <em>entire</em>
contents of <tt class="docutils literal">lib</tt> available without qualification.</p>
</li>
</ol>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() { return 0; }') -->
<!-- @test('run') -->
<p>If we add an additional namespace around keyword declarations, though, we can
give users more control:</p>
<pre class="literal-block">
namespace lib {
    <strong>namespace keywords {</strong>

        BOOST_PARAMETER_NAME(name)
        BOOST_PARAMETER_NAME(index)
    <strong>}</strong>

    BOOST_PARAMETER_FUNCTION(
        (int), f, <strong>keywords::</strong>tag,
        (optional (name,*,&quot;bob&quot;)(index,(int),1))
    )
    {
        std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; index;
        std::cout &lt;&lt; std::endl;
        return index;
    }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
''') -->
<p>Now users need only a single <em>using-directive</em> to bring in just the names of
all keywords associated with <tt class="docutils literal">lib</tt>:</p>
<pre class="literal-block">
<strong>using namespace lib::keywords;</strong>
int y = lib::f(_name = &quot;bob&quot;, _index = 2);
</pre>
<!-- @example.append('int main() { return 0; }') -->
<!-- @test('run', howmany='all') -->
</div>
<div class="section" id="documentation">
<h2><a class="toc-backref" href="#id40">4.3&nbsp;&nbsp;&nbsp;Documentation</a></h2>
<p>The interface idioms enabled by Boost.Parameter are completely new (to C++),
and as such are not served by pre-existing documentation conventions.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This space is empty because we haven't settled on any best practices
yet.  We'd be very pleased to link to your documentation if you've got a
style that you think is worth sharing.</p>
</div>
</div>
</div>
<div class="section" id="portability-considerations">
<h1><a class="toc-backref" href="#id41">5&nbsp;&nbsp;&nbsp;Portability Considerations</a></h1>
<p>Use the <a class="reference external" href="http://www.boost.org/regression/release/user/parameter.html">regression test results</a> for the latest Boost release of
the Parameter library to see how it fares on your favorite
compiler.  Additionally, you may need to be aware of the following
issues and workarounds for particular compilers.</p>
<div class="section" id="perfect-forwarding-support">
<h2><a class="toc-backref" href="#id42">5.1&nbsp;&nbsp;&nbsp;Perfect Forwarding Support</a></h2>
<p>If your compiler supports <a class="reference external" href="http://www.justsoftwaresolutions.co.uk/cplusplus/rvalue_references_and_perfect_forwarding.html">perfect forwarding</a>, then the Parameter library
will <tt class="docutils literal">#define</tt> the macro <tt class="docutils literal">BOOST_PARAMETER_HAS_PERFECT_FORWARDING</tt> unless
you disable it manually.  If your compiler does not provide this support, then
<tt class="docutils literal"><span class="pre">parameter::parameters::operator()</span></tt> will treat rvalue references as lvalue
<tt class="docutils literal">const</tt> references to work around the <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm">forwarding problem</a>, so in certain
cases you must wrap <a class="reference external" href="../../../core/doc/html/core/ref.html"><tt class="docutils literal"><span class="pre">boost::ref</span></tt></a> or <a class="reference external" href="http://en.cppreference.com/w/cpp/utility/functional/ref"><tt class="docutils literal"><span class="pre">std::ref</span></tt></a> around any arguments that will
be bound to out parameters.  The <a class="reference external" href="../../test/evaluate_category.cpp">evaluate_category.cpp</a> and
<a class="reference external" href="../../test/preprocessor_eval_category.cpp">preprocessor_eval_category.cpp</a> test programs demonstrate this support.</p>
</div>
<div class="section" id="boost-mp11-support">
<h2><a class="toc-backref" href="#id43">5.2&nbsp;&nbsp;&nbsp;Boost.MP11 Support</a></h2>
<p>If your compiler is sufficiently compliant with the C++11 standard, then the
Parameter library will <tt class="docutils literal">#define</tt> the macro <tt class="docutils literal">BOOST_PARAMETER_CAN_USE_MP11</tt>
unless you disable it manually.  The <a class="reference external" href="../../test/singular.cpp">singular.cpp</a>, <a class="reference external" href="../../test/compose.cpp">compose.cpp</a>,
<a class="reference external" href="../../test/optional_deduced_sfinae.cpp">optional_deduced_sfinae.cpp</a>, and <a class="reference external" href="../../test/deduced_dependent_predicate.cpp">deduced_dependent_predicate.cpp</a> test programs
demonstrate support for <a class="reference external" href="../../../mp11/doc/html/mp11.html">Boost.MP11</a>.</p>
</div>
<div class="section" id="no-sfinae-support">
<h2><a class="toc-backref" href="#id44">5.3&nbsp;&nbsp;&nbsp;No SFINAE Support</a></h2>
<p>Some older compilers don't support SFINAE.  If your compiler meets that
criterion, then Boost headers will <tt class="docutils literal">#define</tt> the preprocessor symbol
<tt class="docutils literal">BOOST_NO_SFINAE</tt>, and parameter-enabled functions won't be removed
from the overload set based on their signatures.  The <a class="reference external" href="../../test/sfinae.cpp">sfinae.cpp</a> and
<a class="reference external" href="../../test/optional_deduced_sfinae.cpp">optional_deduced_sfinae.cpp</a> test programs demonstrate SFINAE support.</p>
</div>
<div class="section" id="no-support-for-result-of">
<h2>5.4&nbsp;&nbsp;&nbsp;No Support for <a class="reference external" href="../../../utility/utility.htm#result_of"><tt class="docutils literal">result_of</tt></a></h2>
<p><a class="reference internal" href="#lazy-default-computation">Lazy default computation</a> relies on the <tt class="docutils literal">result_of</tt> class template to
compute the types of default arguments given the type of the function object
that constructs them.  On compilers that don't support <tt class="docutils literal">result_of</tt>,
<tt class="docutils literal">BOOST_NO_RESULT_OF</tt> will be <tt class="docutils literal">#define</tt>d, and the compiler will expect
the function object to contain a nested type name, <tt class="docutils literal">result_type</tt>, that
indicates its return type when invoked without arguments.  To use an ordinary
function as a default generator on those compilers, you'll need to wrap it in
a class that provides <tt class="docutils literal">result_type</tt> as a <tt class="docutils literal">typedef</tt> and invokes the
function via its <tt class="docutils literal">operator()</tt>.</p>
<!-- Can't Declare |ParameterSpec| via ``typedef``
=============================================

In principle you can declare a |ParameterSpec| as a ``typedef`` for a
specialization of ``parameters<…>``, but Microsoft Visual C++ 6.x has been
seen to choke on that usage.  The workaround is to use inheritance and
declare your |ParameterSpec| as a class:

.. parsed-literal::

    **struct dfs_parameters
      :** parameter::parameters<
            tag::graph, tag::visitor, tag::root_vertex
          , tag::index_map, tag::color_map
        >
    **{
    };**

Default Arguments Unsupported on Nested Templates
=============================================

As of this writing, Borland compilers don't support the use of default
template arguments on member class templates.  As a result, you have to
supply ``BOOST_PARAMETER_MAX_ARITY`` arguments to every use of
``parameters<…>::match``.  Since the actual defaults used are unspecified,
the workaround is to use |BOOST_PARAMETER_MATCH|_ to declare default
arguments for SFINAE.

.. |BOOST_PARAMETER_MATCH| replace:: ``BOOST_PARAMETER_MATCH`` -->
</div>
<div class="section" id="compiler-can-t-see-references-in-unnamed-namespace">
<h2><a class="toc-backref" href="#id46">5.5&nbsp;&nbsp;&nbsp;Compiler Can't See References In Unnamed Namespace</a></h2>
<p>If you use Microsoft Visual C++ 6.x, you may find that the compiler has
trouble finding your keyword objects.  This problem has been observed, but
only on this one compiler, and it disappeared as the test code evolved, so
we suggest you use it only as a last resort rather than as a preventative
measure.  The solution is to add <em>using-declarations</em> to force the names
to be available in the enclosing namespace without qualification:</p>
<pre class="literal-block">
namespace graphs {

    using graphs::graph;
    using graphs::visitor;
    using graphs::root_vertex;
    using graphs::index_map;
    using graphs::color_map;
}
</pre>
</div>
</div>
<div class="section" id="python-binding">
<h1><a class="toc-backref" href="#id47">6&nbsp;&nbsp;&nbsp;Python Binding</a></h1>
<p>Follow <a class="reference external" href="../../../parameter_python/doc/html/index.html">this link</a> for documentation on how to expose
Boost.Parameter-enabled functions to Python with <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>.</p>
</div>
<div class="section" id="reference">
<h1><a class="toc-backref" href="#id48">7&nbsp;&nbsp;&nbsp;Reference</a></h1>
<p>Follow <a class="reference external" href="reference.html">this link</a> to the Boost.Parameter reference documentation.</p>
</div>
<div class="section" id="glossary">
<h1><a class="toc-backref" href="#id49">8&nbsp;&nbsp;&nbsp;Glossary</a></h1>
<div class="section" id="argument-or-actual-argument">
<span id="arguments"></span><h2><a class="toc-backref" href="#id50">8.1&nbsp;&nbsp;&nbsp;Argument (or “actual argument”)</a></h2>
<p>the value actually passed to a function or class template.</p>
</div>
<div class="section" id="parameter-or-formal-parameter">
<span id="parameter"></span><h2><a class="toc-backref" href="#id51">8.2&nbsp;&nbsp;&nbsp;Parameter (or “formal parameter”)</a></h2>
<p>the name used to refer to an argument within a function or class
template.  For example, the value of <tt class="docutils literal">f</tt>'s <em>parameter</em> <tt class="docutils literal">x</tt> is given by the
<em>argument</em> <tt class="docutils literal">3</tt>:</p>
<pre class="literal-block">
int f(int x) { return x + 1; }
int y = f(3);
</pre>
</div>
</div>
<div class="section" id="acknowledgements">
<h1><a class="toc-backref" href="#id52">9&nbsp;&nbsp;&nbsp;Acknowledgements</a></h1>
<p>The authors would like to thank all the Boosters who participated in the
review of this library and its documentation, most especially our review
manager, Doug Gregor.</p>
<hr class="docutils" />
<table class="docutils footnote" frame="void" id="old-interface" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3">[1]</a></td><td>As of Boost 1.33.0 the Graph library was still using an
<a class="reference external" href="../../../graph/doc/bgl_named_params.html">older named parameter mechanism</a>, but there are plans to change it to
use Boost.Parameter (this library) in an upcoming release, while keeping
the old interface available for backward-compatibility.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="odr" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id5">[2]</a></td><td>The <strong>One Definition Rule</strong> says that any given entity in a C++
program must have the same definition in all translation units (object
files) that make up a program.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="vertex-descriptor" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[3]</td><td>If you're not familiar with the Boost Graph Library,
don't worry about the meaning of any Graph-library-specific details you
encounter.  In this case you could replace all mentions of vertex
descriptor types with <tt class="docutils literal">int</tt> in the text, and your understanding of the
Parameter library wouldn't suffer.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="conceptsts" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id7">[4]</a></td><td>This is a major motivation behind <a class="reference external" href="http://en.cppreference.com/w/cpp/language/constraints">C++20 constraints</a>.</td></tr>
</tbody>
</table>
<!-- .. [#bind] The Lambda library is known not to work on `some -->
<!-- less-conformant compilers`__.  When using one of those you could -->
<!-- use `Boost.Bind`_ to generate the function object\:\: -->
<!-- boost\:\:bind(std\:\:plus<std\:\:string>(),s1,s2) -->
<table class="docutils footnote" frame="void" id="is-keyword-expression" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[5]</td><td><em>(<a class="fn-backref" href="#id13">1</a>, <a class="fn-backref" href="#id14">2</a>)</em> Here we're assuming there's a predicate
metafunction <tt class="docutils literal">is_keyword_expression</tt> that can be used to identify
models of Boost.Python's KeywordExpression concept.</td></tr>
</tbody>
</table>
<!-- .. __ http://www.boost.org/regression/release/user/lambda.html -->
<table class="docutils footnote" frame="void" id="using" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id8">[6]</a></td><td><p class="first">You can always give the illusion that the function
lives in an outer namespace by applying a <em>using-declaration</em>:</p>
<pre class="literal-block">
namespace foo_overloads {

    // foo declarations here
    void foo() { ... }
    ...
}
using foo_overloads::foo;
</pre>
<p class="last">This technique for avoiding unintentional argument-dependent lookup is due
to Herb Sutter.</p>
</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="sfinae" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[7]</td><td>This capability depends on your compiler's support for
SFINAE.  <strong>SFINAE</strong>: <strong>S</strong>ubstitution <strong>F</strong>ailure <strong>I</strong>s <strong>N</strong>ot
<strong>A</strong>n <strong>E</strong>rror.  If type substitution during the instantiation of a
function template results in an invalid type, no compilation error is
emitted; instead the overload is removed from the overload set.  By
producing an invalid type in the function signature depending on the
result of some condition, we can decide whether or not an overload is
considered during overload resolution.  The technique is formalized in the
<a class="reference external" href="../../../core/doc/html/core/enable_if.html"><tt class="docutils literal">enable_if</tt></a> utility.  Most recent compilers support SFINAE; on compilers
that don't support it, the Boost config library will <tt class="docutils literal">#define</tt> the
symbol <tt class="docutils literal">BOOST_NO_SFINAE</tt>.  See
<a class="reference external" href="http://www.semantics.org/once_weakly/w02_SFINAE.pdf">http://www.semantics.org/once_weakly/w02_SFINAE.pdf</a> for more information
on SFINAE.</td></tr>
</tbody>
</table>
</div>
</div>
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